Solve the Logarithmic Inequality: log₄x × log₅64 ≥ log₅(x³+x²+x+1)

Logarithmic Inequalities with Change of Base

Given 0

log4x×log564log5(x3+x2+x+1) \log_4x\times\log_564\ge\log_5(x^3+x^2+x+1)

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:16 Let's solve this problem together.
00:21 We'll start by using the logarithm product formula. We may need to change between different bases, so keep that in mind.
00:33 Next, let's solve the given logarithm. Then we can substitute the result back into the exercise. Ready? Here we go.
01:03 We'll apply the power rule for logarithms. This means raising the number by the coefficient.
01:13 Now, let's compare the numbers in the logarithm. Doing great so far!
01:23 Let's simplify what we can. Reducing is like cleaning up your work and making it easier to see.
01:33 We'll use the root formula to find possible solutions. Keep in mind what we discussed about roots.
01:39 Remember, there are no roots less than zero.
01:43 Therefore, there is no solution to this question. And that's how we solve it!

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

Given 0

log4x×log564log5(x3+x2+x+1) \log_4x\times\log_564\ge\log_5(x^3+x^2+x+1)

2

Step-by-step solution

To solve this problem, we need to compare the expressions log4x×log564 \log_4 x \times \log_5 64 and log5(x3+x2+x+1)\log_5 (x^3 + x^2 + x + 1).

First, calculate log564 \log_5 64 . We know that 64=43=26 64 = 4^3 = 2^6 . Therefore:
log564=log526log54=6log522log52=3 \log_5 64 = \frac{\log_5 2^6}{\log_5 4} = \frac{6 \log_5 2}{2 \log_5 2} = 3

Next, simplify the left-hand side expression log4x \log_4 x . Using the change of base formula:
log4x=log5xlog54 \log_4 x = \frac{\log_5 x}{\log_5 4}

Therefore, the left-hand side becomes:
log5xlog54×3=3log5x2log52 \frac{\log_5 x}{\log_5 4} \times 3 = \frac{3 \log_5 x}{2 \log_5 2}

For the inequality:
3log5x2log52log5(x3+x2+x+1) \frac{3 \log_5 x}{2 \log_5 2} \ge \log_5 (x^3 + x^2 + x + 1)

We can now equate the right-hand side:
log5x3/2log52log5(x3+x2+x+1) \log_5 x^{3/2\log_5 2} \ge \log_5 (x^3 + x^2 + x + 1)

This implies:
x3/2log52x3+x2+x+1 x^{3/2\log_5 2} \ge x^3 + x^2 + x + 1

Testing and analyzing this expression results in no valid x x satisfying the inequality within real values since exponential growth and polynomial terms do not align. Thus, the inequality cannot be satisfied, and no solution satisfies the given conditions.

Therefore, the solution to the problem is: No solution.

3

Final Answer

No solution

Key Points to Remember

Essential concepts to master this topic
  • Domain: All logarithmic expressions must have positive arguments
  • Technique: Convert log₄x = log₅x/log₅4 using change of base formula
  • Check: Test boundary values and verify domain restrictions are satisfied ✓

Common Mistakes

Avoid these frequent errors
  • Ignoring domain restrictions when solving logarithmic inequalities
    Don't solve log₄x ≥ some expression without checking that x > 0 first = undefined logarithms! This creates invalid solutions that don't exist in reality. Always verify the domain of every logarithmic term before and after solving.

Practice Quiz

Test your knowledge with interactive questions

\( \log_75-\log_72= \)

FAQ

Everything you need to know about this question

Why does this inequality have no solution even though x > 0?

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Even with x > 0, the inequality x3/2log52x3+x2+x+1 x^{3/2\log_5 2} \ge x^3 + x^2 + x + 1 cannot be satisfied! The left side grows much slower than the polynomial on the right for any positive x value.

How do I calculate log₅64 without a calculator?

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Since 64 = 4³ and we need it in terms of base 5, use: log564=log543=3log54 \log_5 64 = \log_5 4^3 = 3\log_5 4 . This simplifies our inequality significantly!

What's the change of base formula and when do I use it?

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The formula is logax=logbxlogba \log_a x = \frac{\log_b x}{\log_b a} . Use it when you have different bases in the same problem, like log₄x and log₅ terms here.

Can logarithmic inequalities ever have no solution?

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Yes! Unlike simple equations, inequalities can have no solution when the expressions never satisfy the inequality condition, even within valid domains.

How do I factor x³ + x² + x + 1?

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Group the terms: x3+x2+x+1=x2(x+1)+1(x+1)=(x2+1)(x+1) x^3 + x^2 + x + 1 = x^2(x + 1) + 1(x + 1) = (x^2 + 1)(x + 1) . This factoring can help analyze the inequality behavior.

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