Another one of those inequalities

Algebra Level 3

Solve log 5 ( 3 2 x ) log 5 ( 4 x + 1 ) . \log_5(3-2x) \ge \log_5(4x+1).

If x ( a , b ] x \in (-a,b] , then find a × b a \times b .

Give your answer to 3 decimal places.


This question is not an original.


The answer is 0.083.

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1 solution

Rishik Jain
Feb 19, 2016

Well, first of all, 3 2 x > 0 4 x + 1 > 0 x ( 1 4 , 2 3 ) 3-2x > 0 \\ 4x+1>0 \\ \therefore x \in \left(-\frac{1}{4},\frac{2}{3}\right) Now, log 5 ( 3 2 x ) log 5 ( 4 x 1 ) 3 2 x 4 x 1 6 x 2 x 1 3 \log_5(3-2x) \ge \log_5(4x-1) \\ 3-2x \ge 4x-1 \\ 6x \le 2 \\ x \le \frac{1}{3} Combining these inequalities, we get x ( 1 4 , 1 3 ] a = 1 4 and b = 1 3 a × b = 1 12 = 0.083 x \in (-\frac{1}{4},\frac{1}{3}] \\ a=\frac{1}{4} \text{ and } b=\frac{1}{3} \\ a \times b= \frac{1}{12} = \boxed{0.083}

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