Math Problem Statement

3 * log_5(x - 4) > 6/(log_5(x - 4) + 1)

Solution

Let's solve the inequality:

3log5(x4)>6log5(x4)+1.3 \cdot \log_5(x - 4) > \frac{6}{\log_5(x - 4) + 1}.


Step 1: Let y=log5(x4)y = \log_5(x - 4)

This substitution simplifies the inequality. Recall that log5(x4)\log_5(x - 4) is valid only when x4>0x - 4 > 0, so:

x>4.x > 4.

The inequality becomes:

3y>6y+1.3y > \frac{6}{y + 1}.


Step 2: Eliminate the fraction

Multiply both sides by y+1y + 1, assuming y1y \neq -1 (to avoid division by zero):

3y(y+1)>6.3y(y + 1) > 6.

Simplify:

3y2+3y6>0.3y^2 + 3y - 6 > 0.


Step 3: Solve the quadratic inequality

Divide through by 3:

y2+y2>0.y^2 + y - 2 > 0.

Factorize the quadratic:

(y+2)(y1)>0.(y + 2)(y - 1) > 0.

The critical points are y=2y = -2 and y=1y = 1. Using a sign test, determine where the inequality is true:

  • For y<2y < -2, (y+2)(y1)>0(y + 2)(y - 1) > 0: false (both terms are negative).
  • For 2<y<1-2 < y < 1, (y+2)(y1)>0(y + 2)(y - 1) > 0: false (one term is positive, the other is negative).
  • For y>1y > 1, (y+2)(y1)>0(y + 2)(y - 1) > 0: true (both terms are positive).

Thus, the solution for yy is:

y>1.y > 1.


Step 4: Back-substitute y=log5(x4)y = \log_5(x - 4)

We now solve:

log5(x4)>1.\log_5(x - 4) > 1.

Recall that log5(a)>b\log_5(a) > b implies a>5ba > 5^b. Therefore:

x4>51x4>5x>9.x - 4 > 5^1 \quad \Rightarrow \quad x - 4 > 5 \quad \Rightarrow \quad x > 9.


Step 5: Combine with initial condition

Initially, we had x>4x > 4. Combining this with x>9x > 9, the solution is:

x>9.x > 9.


Final Answer:

x>9.\boxed{x > 9.}


Let me know if you'd like a detailed explanation of any step or if you have more questions!

Related Questions:

  1. What are the properties of logarithmic inequalities?
  2. How do you perform a sign test for quadratic inequalities?
  3. How do you handle equations with multiple logarithms?
  4. Can logarithmic equations have extraneous solutions?
  5. How do transformations of logarithmic graphs affect inequality solutions?

Tip: Always check the domain of a logarithmic inequality before solving to ensure all expressions are valid!

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Math Problem Analysis

Mathematical Concepts

Logarithmic Inequalities
Quadratic Inequalities
Logarithmic Functions
Inequalities

Formulas

log_b(a) > c ==> a > b^c
ax^2 + bx + c = 0 (Quadratic Equation)
Sign Test for Quadratic Inequalities

Theorems

Logarithmic Properties
Quadratic Inequality Solutions

Suitable Grade Level

Grades 9-12