Log 10 4x Log 10 3x: Exact Answer & Steps

23 min read

Ever tried to untangle something like log₁₀(4x) · log₁₀(3x) and felt the brain melt?
You’re not alone. Most of us have stared at a pair of logs, wondered whether we should multiply them, add them, or just throw the whole thing out. The short version is: those two logs hide a lot of useful algebraic tricks, and once you see them, the rest falls into place.


What Is log₁₀(4x) · log₁₀(3x)

At its core, log₁₀(4x) · log₁₀(3x) is simply the product of two common‑logarithm expressions. “Common log” just means the base is 10, the same base you see on calculators when you hit the “log” button.

So we have:

  • log₁₀(4x) – the power you raise 10 to in order to get 4x.
  • log₁₀(3x) – the power you raise 10 to in order to get 3x.

Multiplying them doesn’t magically turn them into a single log. Instead, the product invites us to use logarithmic identities, factor out constants, and sometimes even solve equations that involve the whole expression Which is the point..

Where the “4x” and “3x” Come From

The 4x and 3x are just linear terms. They could be any positive expression—5y, 7z, 0.Plus, 2t—but the numbers 4 and 3 are nice because they’re small, co‑prime, and they give us a clean way to illustrate the rules. Remember: logs only like positive arguments, so x itself must be positive for the whole expression to make sense.

Real talk — this step gets skipped all the time.


Why It Matters / Why People Care

You might wonder why anyone would waste time on a product of two logs. The truth is, this kind of expression pops up in a few real‑world spots:

  • Engineering calculations – when dealing with decibel scales, you often multiply logs to combine gains or losses.
  • Finance – compound interest formulas sometimes involve products of logs when you solve for time or rate.
  • College algebra – exam questions love to hide a simple substitution behind a messy-looking product.

If you can simplify log₁₀(4x) · log₁₀(3x) quickly, you’ll shave minutes off a test, avoid a calculation error in a spreadsheet, and—honestly—feel a little smarter Nothing fancy..


How It Works (or How to Do It)

Below is the toolbox you need. In real terms, each tool is a property of logarithms or a simple algebraic step. Grab the one that fits your problem.

1. Break Down Each Log Using the Product Rule

The product rule says log₁₀(ab) = log₁₀ a + log₁₀ b. Apply it to both factors:

log₁₀(4x) = log₁₀ 4 + log₁₀ x
log₁₀(3x) = log₁₀ 3 + log₁₀ x

Now the original product becomes:

(log₁₀ 4 + log₁₀ x) · (log₁₀ 3 + log₁₀ x)

2. Expand Like a Quadratic

Treat the expression as (A + B)(C + D). Multiply it out:

= (log₁₀ 4)(log₁₀ 3) 
  + (log₁₀ 4)(log₁₀ x) 
  + (log₁₀ 3)(log₁₀ x) 
  + (log₁₀ x)²

That’s a tidy polynomial in log₁₀ x. If you need a numeric answer, just plug the known values:

  • log₁₀ 4 ≈ 0.60206
  • log₁₀ 3 ≈ 0.47712

So the constant term (log₁₀ 4)(log₁₀ 3) is about 0.287 Worth knowing..

3. Recognize a Common Factor

Both middle terms contain log₁₀ x. Factor it:

= (log₁₀ 4)(log₁₀ 3) 
  + log₁₀ x [log₁₀ 4 + log₁₀ 3] 
  + (log₁₀ x)²

Now you see a perfect square structure:

= (log₁₀ x)² + (log₁₀ 4 + log₁₀ 3)·log₁₀ x + (log₁₀ 4)(log₁₀ 3)

That’s exactly the quadratic form ax² + bx + c with a = 1, b = log₁₀ 4 + log₁₀ 3, c = (log₁₀ 4)(log₁₀ 3) Practical, not theoretical..

4. Solve an Equation Involving the Product

Suppose you’re asked to find x such that:

log₁₀(4x)·log₁₀(3x) = 1

Replace the product with the quadratic form we just derived:

(log₁₀ x)² + (log₁₀ 4 + log₁₀ 3)·log₁₀ x + (log₁₀ 4)(log₁₀ 3) - 1 = 0

Now it’s a standard quadratic in log₁₀ x. Use the quadratic formula:

log₁₀ x = [-(log₁₀ 4 + log₁₀ 3) ± √{(log₁₀ 4 + log₁₀ 3)² - 4·1·[(log₁₀ 4)(log₁₀ 3) - 1]}] / 2

Plug the numbers, simplify, then exponentiate (x = 10^{log₁₀ x}) to get the actual x values. The process feels messy, but the structure is clean—thanks to the expansion step.

5. Use Change‑of‑Base if You Want a Different Base

Sometimes a problem prefers natural logs (ln) or base‑2 logs. The change‑of‑base formula says:

log₁₀(4x) = ln(4x) / ln 10
log₁₀(3x) = ln(3x) / ln 10

Multiplying gives:

[ln(4x)·ln(3x)] / (ln 10)²

Now the denominator is just a constant (≈ 5.Worth adding: 302). If you’re working in a programming language that only has ln, this version is the one to code.

6. Graphical Insight

If you plot y = log₁₀(4x)·log₁₀(3x) for x > 0, you’ll see a smooth curve that dips near x = 0.Even so, 1 and climbs slowly after x = 1. That said, the shape mirrors the quadratic in log₁₀ x. Knowing the curve helps you guess where a solution to y = k might lie before you even start algebra Worth keeping that in mind..


Common Mistakes / What Most People Get Wrong

  1. Treating the product as a single log.
    log₁₀(4x)·log₁₀(3x) ≠ log₁₀(12x²). The product rule works for sums of logs, not for their multiplication.

  2. Forgetting the domain.
    Both 4x and 3x must be positive, so x > 0. It’s easy to overlook that and end up with an extraneous negative root after solving a quadratic.

  3. Dropping the constant term when expanding.
    The (log₁₀ 4)(log₁₀ 3) piece is tiny but not zero. Skipping it throws off any numeric answer.

  4. Mixing bases unintentionally.
    If you write log(4x) in a spreadsheet, it might default to natural log. That changes the whole expression unless you divide by ln 10.

  5. Assuming symmetry.
    Because the numbers 3 and 4 are close, some think the expression is symmetric in x. It isn’t; the log₁₀ x terms dominate as x grows Simple as that..


Practical Tips / What Actually Works

  • Always rewrite each log with the product rule first. It separates the constant part from the variable part and makes the rest of the work painless.
  • Keep a small table of common log values (log₁₀ 2 ≈ 0.3010, log₁₀ 3 ≈ 0.4771, log₁₀ 4 ≈ 0.6021). You’ll rarely need a calculator for the constants.
  • When solving equations, treat log₁₀ x as a single unknown (let y = log₁₀ x). The quadratic becomes y² + by + c = 0, which is trivial to handle.
  • Check the domain after you solve. Plug each candidate x back into the original product; if any argument is ≤ 0, discard it.
  • If you need a quick numeric estimate, use the approximation log₁₀(4x)·log₁₀(3x) ≈ (0.6 + log₁₀ x)(0.48 + log₁₀ x). Multiply out and you have a decent mental calculator.
  • For programming, compute the logs once, store them, then reuse. Re‑calculating log₁₀ x twice is wasteful.

FAQ

Q: Can I simplify log₁₀(4x)·log₁₀(3x) into a single log with a different base?
A: Not directly. You can rewrite each factor using the change‑of‑base formula, which gives you a product of natural logs divided by a constant. That’s as close as you get to a “single” expression Worth keeping that in mind..

Q: What if x is a fraction, like x = 0.05?
A: The same steps apply. Just remember that log₁₀ x will be negative, so the quadratic may have a different sign pattern, but the algebra holds It's one of those things that adds up..

Q: Is there a neat identity for log₁₀(a·x)·log₁₀(b·x) in general?
A: Yes. Expand to (log₁₀ a + log₁₀ x)(log₁₀ b + log₁₀ x) = (log₁₀ x)² + (log₁₀ a + log₁₀ b)·log₁₀ x + (log₁₀ a)(log₁₀ b). It’s a universal quadratic in log₁₀ x.

Q: How do I handle this expression in Excel?
A: Use =LOG10(4*A1)*LOG10(3*A1). If you need natural logs, replace with =LN(4*A1)*LN(3*A1)/POWER(LN(10),2).

Q: Does the product ever become zero?
A: Only if one of the logs is zero, i.e., log₁₀(4x)=0 or log₁₀(3x)=0. That happens when 4x = 1 (so x = 0.25) or 3x = 1 (x ≈ 0.333). At those points the product is indeed zero.


That’s the whole picture: break the logs apart, treat the variable part as a single unknown, and you’ll tame even the most intimidating looking log₁₀(4x)·log₁₀(3x). Next time you see it on a worksheet or a test, you’ll know exactly where to start—and you’ll finish with confidence. Happy calculating!

Going Beyond the Basics

1. The Logarithm as a Function of x

When you see an expression like

[ \log_{10}(4x),\log_{10}(3x), ]

think of it as a function (f(x)). Its shape is a parabola in the variable (y=\log_{10}x). So because logarithms are continuous and strictly increasing on ((0,\infty)), the domain of (f) is exactly the set of positive (x) for which both arguments stay positive, i. e. (x>0). No hidden tricks—just the ordinary rules of logs.

2. A Quick “Rule of Thumb” for Growth

If you only need to know whether the product grows or shrinks as (x) increases, you can ignore the constant terms:

[ f(x)\approx (\log_{10}x)^2. ]

Since (\log_{10}x) is positive for (x>1) and negative for (0<x<1), the sign of (f(x)) flips at (x=1). For (x>1) the product is positive and grows quadratically with (\log_{10}x); for (0<x<1) it’s positive too because a negative times a negative gives a positive. The only place it turns zero is when one of the factors hits zero, which happens exactly at (x=1/4) and (x=1/3) Worth keeping that in mind..

3. Scaling and Shifting

If you replace (x) by (kx) where (k>0), the transformation is

[ \log_{10}(4kx),\log_{10}(3kx) = \bigl(\log_{10}k+\log_{10}4+\log_{10}x\bigr) \bigl(\log_{10}k+\log_{10}3+\log_{10}x\bigr). ]

So scaling (x) by a constant just shifts the quadratic in (y=\log_{10}x) horizontally. This is handy when you want to compare data sets that differ by a constant factor.

4. Inverting the Function

Suppose you’re given a value (c) and asked to solve

[ \log_{10}(4x),\log_{10}(3x)=c. ]

Set (y=\log_{10}x) and solve the quadratic

[ y^2+0.4810,y+0.2878-c=0. ]

The solutions are

[ y=\frac{-0.4810\pm\sqrt{0.4810^2-4(0.2878-c)}}{2}, ]

and then (x=10^{,y}). The discriminant must be non‑negative; otherwise no real (x) satisfies the equation And that's really what it comes down to..

5. Special Cases Worth Remembering

Case Expression Result
(x=1) (\log_{10}4\cdot\log_{10}3) (0.Worth adding: 6021\cdot0. 4771\approx0.

These quick checks are useful when you’re sketching the graph or verifying boundary conditions.


Final Words

The product (\log_{10}(4x)\cdot\log_{10}(3x)) looks intimidating at first glance, but once you peel it back layer by layer it reveals a clean quadratic in (\log_{10}x). By:

  1. Expanding each logarithm with the product rule,
  2. Treating (\log_{10}x) as a single variable, and
  3. Applying the usual quadratic‑equation techniques,

you can solve, analyze, and graph the expression with ease. The same strategy works for any product of the form (\log_{10}(a x),\log_{10}(b x)) or even for natural logs—just replace the base constants accordingly Which is the point..

So the next time you encounter a product of logs, remember: it’s just a parabola in disguise. Here's the thing — break it out, solve it, and you’ll have the answer in no time. Happy logarithming!

6. Extending to Other Bases

The analysis above hinged on the fact that the logarithm base was 10, but the same steps work for any base (b>0,;b\neq1). Writing the product as

[ \log_{b}(4x),\log_{b}(3x) ]

and using (\log_{b}(kx)=\log_{b}k+\log_{b}x) gives

[ \bigl(\log_{b}4+\log_{b}x\bigr)\bigl(\log_{b}3+\log_{b}x\bigr) = (\log_{b}x)^{2}+(\log_{b}4+\log_{b}3),\log_{b}x+\log_{b}4;\log_{b}3. ]

Thus the coefficients are simply (\log_{b}4+\log_{b}3) and (\log_{b}4,\log_{b}3). If you prefer natural logs, replace each (\log_{b}) with (\ln) and the algebra proceeds unchanged. The only practical difference is the numerical values of the constants; the shape of the graph (a parabola in the (\log_{b}x)‑plane) is identical for every base Worth knowing..

7. Derivatives and Critical Points

Every time you need the rate of change of the product with respect to (x), differentiate directly:

[ \frac{d}{dx}\bigl[\log_{10}(4x),\log_{10}(3x)\bigr] = \frac{1}{x\ln 10}\Bigl[\log_{10}(4x)+\log_{10}(3x)\Bigr]. ]

Setting the derivative equal to zero yields

[ \log_{10}(4x)+\log_{10}(3x)=0\quad\Longrightarrow\quad \log_{10}(12x^{2})=0;\Longrightarrow;x=\frac{1}{\sqrt{12}}. ]

That value, (x=1/\sqrt{12}\approx0.2887), is the unique stationary point of the product. Substituting back gives the minimum value

[ \bigl[\log_{10}(4x),\log_{10}(3x)\bigr]{!x=1/\sqrt{12}} = -\frac{(\log{10}12)^{2}}{4}\approx-0.036. ]

Because the quadratic in (\log_{10}x) opens upward (its leading coefficient is (+1)), this stationary point is a global minimum. The function therefore dips slightly below the axis between the two zeros at (x=1/4) and (x=1/3), then rises without bound on either side Small thing, real impact..

Quick note before moving on.

8. Integrals Involving the Product

A common calculus exercise asks for

[ \int \log_{10}(4x),\log_{10}(3x),dx. ]

Using the substitution (y=\log_{10}x) ((x=10^{y}), (dx=10^{y}\ln 10,dy)) transforms the integral into

[ \int \bigl(y+\log_{10}4\bigr)\bigl(y+\log_{10}3\bigr),10^{y}\ln 10,dy. ]

Expanding the quadratic and integrating term‑by‑term yields

[ \ln 10\int!\bigl(y^{2}+(\log_{10}4+\log_{10}3)y+\log_{10}4,\log_{10}3\bigr)10^{y},dy. ]

Each piece is of the form (\int y^{k}10^{y},dy), which can be evaluated by repeated integration by parts or by recognizing the generating function

[ \int y^{k}a^{y},dy = \frac{a^{y}}{\ln a}\Bigl(y^{k}-\frac{k}{\ln a}y^{k-1}+\frac{k(k-1)}{(\ln a)^{2}}y^{k-2}-\cdots\Bigr)+C. ]

Carrying this out for (k=0,1,2) and substituting back (y=\log_{10}x) gives a closed‑form antiderivative; the expression is messy but completely elementary. In practice, numerical integration is often faster when a specific interval is required Nothing fancy..

9. Applications and Why It Matters

Although the expression (\log_{10}(4x),\log_{10}(3x)) may appear as a textbook curiosity, it surfaces in several applied contexts:

Field Typical Appearance Reason
Signal processing Power spectral density calculations involving decibel scales Decibel values are (10\log_{10}) of a ratio; products of two such terms arise when cascading filters
Economics Elasticity models where two logarithmic demand functions are multiplied Log‑elasticities add linearly; the product captures interaction effects
Chemistry pH‑related equilibria with mixed acid–base systems (pH=-\log_{10}[H^+]); products of log terms appear when coupling two equilibria
Computer science Complexity analysis of algorithms with nested logarithmic factors Running time may be expressed as (\log n \cdot \log (cn)) for constant (c)

In each case, recognizing the underlying quadratic in (\log x) simplifies both analytic work and numerical estimation.

10. A Quick Checklist for Working with (\log_{10}(4x),\log_{10}(3x))

  1. Domain – Ensure (x>0).
  2. Zeros – Solve (\log_{10}(4x)=0) and (\log_{10}(3x)=0) → (x=1/4,,1/3).
  3. Sign – Positive for (x<1/4) and (x>1/3); negative between the zeros.
  4. Extrema – Derivative zero at (x=1/\sqrt{12}); this is the global minimum.
  5. Scaling – Replacing (x) by (kx) shifts the quadratic horizontally by (\log_{10}k).
  6. Base change – Replace 10 with any base (b); the algebraic form stays the same.
  7. Solving for (x) – Convert to a quadratic in (y=\log_{10}x), solve, then exponentiate.

Conclusion

The product (\log_{10}(4x),\log_{10}(3x)) is far less mysterious than its notation suggests. Consider this: by pulling the logarithmic identities apart, we uncover a simple quadratic in the variable (\log_{10}x). This insight unlocks every standard tool—zero‑finding, differentiation, integration, and scaling—making the expression tractable whether you are sketching a graph, optimizing a model, or solving an equation.

Remember: whenever you see a product of two logs that share the same argument structure, rewrite each log as a constant plus (\log) of the variable. The resulting expression will always be a parabola in the log‑space, and all the familiar properties of quadratics will follow. Because of that, armed with this perspective, you can turn what initially looks like a cumbersome algebraic obstacle into a straightforward, elegant calculation. Happy problem‑solving!

11. Extending the Pattern to Arbitrary Constants

The same trick works when the constants 4 and 3 are replaced by any positive numbers (a) and (b): [ f_{a,b}(x)=\log_{10}(ax),\log_{10}(bx). The vertex is located at [ y_{v}=-\tfrac12(\log_{10}a+\log_{10}b), \qquad x_{v}=10^{y_{v}}=\frac{1}{\sqrt{ab}}, ] and the minimum value is [ f_{a,b}(x_{v})=-\tfrac14(\log_{10}a-\log_{10}b)^{2}\le 0. ] Setting (y=\log_{10}x) yields [ f_{a,b}(x)=\bigl(\log_{10}a+y\bigr)\bigl(\log_{10}b+y\bigr) =y^{2}+y(\log_{10}a+\log_{10}b)+\log_{10}a,\log_{10}b. Which means ] Thus the quadratic’s coefficients are simply the logarithms of the constants. ] When (a=b) the minimum collapses to zero at (x=1/a), reflecting the symmetry of the two logs Small thing, real impact..

12. Numerical Approximation Techniques

In practice one often needs a quick numerical value of (f_{4,3}(x)) for a specific (x). Two inexpensive methods are:

  1. Linear interpolation on the log‑scale.
    Compute (y=\log_{10}x), evaluate the quadratic (y^{2}+5y+2) directly.
    This requires only one logarithm and a few multiplications—ideal for embedded systems.

  2. Taylor series about the vertex.
    For (x) close to (1/\sqrt{12}), write (x=(1/\sqrt{12}),e^{t}) with small (t).
    Then (\log_{10}x = \log_{10}(1/\sqrt{12})+t/\ln 10), and [ f_{4,3}(x)=\left(\log_{10}\frac{4}{\sqrt{12}}+\frac{t}{\ln 10}\right) \left(\log_{10}\frac{3}{\sqrt{12}}+\frac{t}{\ln 10}\right). ] Expanding to second order in (t) gives a highly accurate estimate with only elementary arithmetic.

13. Symbolic Computation Tips

When using computer algebra systems (CAS) to manipulate expressions of this type, the following tricks save time:

  • Factor the log arguments: log(4*x)log(4)+log(x) before multiplying.
  • Collect terms in log(x): this automatically produces the quadratic form.
  • Use solve on the quadratic: most CAS will return the exact roots (x=1/4,,1/3) without manual substitution.
  • Plot with plot: supplying the domain (x>0) and the range of interest gives a clear visual of the parabola in log‑space.

14. Real‑World Scenario: Cascaded Filter Design

In digital signal processing, two first‑order low‑pass filters with cut‑off frequencies (f_{c1}) and (f_{c2}) are often cascaded. The overall transfer function magnitude at a test frequency (f) is [ |H(f)|=\frac{1}{\sqrt{1+(f/f_{c1})^{2}}}\cdot\frac{1}{\sqrt{1+(f/f_{c2})^{2}}}. ] Expressing the attenuation in decibels: [ A(f)=-20\log_{10}|H(f)| =10\log_{10}!\bigl(1+(f/f_{c1})^{2}\bigr) +10\log_{10}!Still, \bigl(1+(f/f_{c2})^{2}\bigr). ] If (f_{c1}) and (f_{c2}) are chosen such that (f_{c1}=4f_{0}) and (f_{c2}=3f_{0}), the two logarithmic terms become exactly the form studied earlier, with (x=f/f_{0}). In practice, the minimum attenuation occurs when (x=1/\sqrt{12}), i. e., at a frequency (\frac{1}{\sqrt{12}},f_{0}), a fact that can be used to tune the filter for a target ripple level.


Final Thoughts

The seemingly innocuous product (\log_{10}(4x),\log_{10}(3x)) is a microcosm of a broader theme: logarithmic expressions that share a common variable can often be collapsed into simple algebraic forms when the properties of the logarithm are exploited. By rewriting each log as a constant plus the log of the variable, the product becomes a quadratic in (\log_{10}x). This single transformation unlocks a host of analytical tools—exact zeros, extrema, integrals, and scalable behavior—that would otherwise be obscured Worth knowing..

Whether you are a student wrestling with an algebra problem, an engineer optimizing a signal chain, or a scientist modeling chemical equilibria, remember that turning a product of logs into a quadratic is usually the first step toward clarity. Once the quadratic is in hand, the rest of the analysis follows in a familiar, systematic way. Happy exploring!

15. Extending the Idea to Other Bases

The derivation above relied on base‑10 logarithms because the original problem was posed in that base, but the same technique works for any base (b>0), (b\neq1). Let

[ L_b(x)=\log_b(4x),\log_b(3x). ]

Using the change‑of‑base formula (\log_b y=\dfrac{\ln y}{\ln b}) we obtain

[ L_b(x)=\frac{1}{(\ln b)^2},\bigl[\ln(4)+\ln x\bigr]\bigl[\ln(3)+\ln x\bigr]. ]

Thus the quadratic in (\ln x) is simply scaled by the constant factor (1/(\ln b)^2). Consequently:

Base (b) Quadratic coefficient Roots (in (x)) Vertex (in (\log_b x))
10 1 (\frac14,\frac13) (-\frac{\ln 12}{2\ln 10})
(e) 1 (\frac14,\frac13) (-\frac{\ln 12}{2})
2 (\bigl(\ln 2\bigr)^{-2}) (\frac14,\frac13) (-\frac{\ln 12}{2\ln 2})

You'll probably want to bookmark this section That's the part that actually makes a difference. And it works..

The shape of the parabola is unchanged; only the vertical scaling varies with the base. This observation is useful when the problem originates in information theory (base‑2 logs) or in natural‑science contexts (base‑(e) logs).

16. A Probabilistic Interpretation

Consider a random variable (X) that takes the values (\frac14) and (\frac13) with equal probability. Its probability mass function is

[ P(X=x)=\frac12\bigl[\delta(x-\tfrac14)+\delta(x-\tfrac13)\bigr]. ]

If we define a new random variable

[ Y=\log_{10}(4X),\log_{10}(3X), ]

then (Y) takes exactly the two values computed in Section 5:

[ Y\in\Bigl{,\log_{10}!4,\log_{10}!3,; \bigl(\log_{10}4-2\bigr)\bigl(\log_{10}3-2\bigr)\Bigr}. ]

The expected value of (Y) is simply the arithmetic mean of those two numbers:

[ \mathbb{E}[Y]=\frac12\Bigl[\log_{10}!4,\log_{10}!3+ \bigl(\log_{10}4-2\bigr)\bigl(\log_{10}3-2\bigr)\Bigr] \approx-0.129. ]

While this construction may appear contrived, it illustrates how the product of logs can serve as a statistical functional—a mapping from a distribution of (X) to a single scalar that captures interaction between two scaling factors (4 and 3). In more elaborate models, such functionals appear in entropy calculations and in the analysis of multiplicative noise.

17. Generalising to Multiple Factors

Suppose we have (n) linear factors (a_i x) with positive constants (a_i). The product of their logarithms can be written as

[ \prod_{i=1}^{n}\log_{10}(a_i x) =\prod_{i=1}^{n}\bigl(\log_{10}a_i+\log_{10}x\bigr). ]

Expanding this product yields a polynomial of degree (n) in (\log_{10}x). For (n=2) we recover the quadratic discussed throughout the article; for (n=3) we obtain a cubic, and so on. The coefficients are elementary symmetric functions of the constants (\log_{10}a_i). The same ideas—identifying zeros, extrema (via derivatives), and asymptotic behaviour—extend naturally, although the algebra becomes richer.

A particularly elegant case occurs when the constants form a geometric progression: (a_i = r^{i-1}). Then

[ \log_{10}(a_i x)=\log_{10}x+(i-1)\log_{10}r, ]

and the product simplifies to

[ \bigl(\log_{10}x\bigr)^n +\binom{n}{1}\bigl(\log_{10}r\bigr)\bigl(\log_{10}x\bigr)^{n-1} +\dots+\bigl(\log_{10}r\bigr)^{n}. ]

Basically precisely the expansion of (\bigl(\log_{10}x+\log_{10}r\bigr)^{n}), confirming the intuition that a geometric sequence of multipliers behaves like a single shifted log raised to the (n)‑th power Easy to understand, harder to ignore..

18. Pedagogical Take‑aways

  1. Always isolate the variable inside the logarithm – factor constants first.
  2. Convert to a single log base – this reduces the problem to algebraic manipulation.
  3. Recognise the quadratic structure – once the product is expressed as (A(\log x)^2+B\log x+C), all familiar tools (completing the square, discriminant, derivative) become available.
  4. Check domain restrictions early – the argument of each log must stay positive, which translates into simple interval constraints on (x).
  5. Use technology wisely – CAS can automate expansion and factorisation, but understanding the underlying steps prevents mis‑interpretation of symbolic output.

19. A Quick “Cheat Sheet”

Goal Transformation Result
Write product as quadratic (\log(4x)=\log4+\log x), (\log(3x)=\log3+\log x) ((\log x)^2+(\log4+\log3)\log x+\log4\log3)
Find zeros Solve (\log x = -\log4) or (\log x = -\log3) (x=1/4,;x=1/3)
Locate vertex (\log x = -\frac{\log4+\log3}{2}) (x=10^{-(\log4+\log3)/2}=1/\sqrt{12})
Evaluate at vertex Substitute back (-\frac{1}{4}(\log4-\log3)^2)
Integrate (\int (\log x)^2dx = x[(\log x)^2-2\log x+2]) Add constant term ((\log4+\log3)\int\log x,dx) etc.

Having such a compact reference at hand accelerates problem‑solving in timed examinations or rapid‑prototype coding.


Conclusion

The product (\log_{10}(4x),\log_{10}(3x)) is more than a textbook exercise; it is a gateway to a suite of techniques that bridge elementary algebra, calculus, and applied mathematics. By pulling the constants out of the logarithms, the expression collapses into a quadratic in (\log_{10}x), a form that instantly reveals its zeros, extremum, and integral. This quadratic perspective persists across bases, scales to multiple factors, and even admits probabilistic and engineering interpretations.

In practice, the same pattern recurs whenever several logarithmic terms share a common variable—whether in filter design, chemical equilibria, or information‑theoretic measures. Plus, recognising the hidden quadratic early saves effort, prevents algebraic mishaps, and opens the door to powerful analytical shortcuts. Armed with the systematic approach outlined above, you can tackle any analogous log‑product with confidence, turning a potentially messy expression into a clean, tractable problem.

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