Logging inequalities 2

Algebra Level 3

Solve the inequation: log [ x 2 12 x + 30 10 ] [ log 2 2 x 5 ] > 0 \log_{\left[\dfrac{x^2-12x+30}{10}\right]}{\left[\log_{2}{\dfrac{2x}{5}}\right]}>0

x ( 5 3 , 6 6 ) ( 10 , ) x \in (\dfrac{5}{3},6-\sqrt{6}) \cup (10,\infty) x ( 5 2 , 6 6 ) ( 1 , ) x \in (\dfrac{5}{2},6-\sqrt{6}) \cup (1,\infty) x ( 5 2 , 6 6 ) ( 10 , ) x \in (\dfrac{5}{2},6-\sqrt{6}) \cup (10,\infty) x ( 4 3 , 5 6 ) ( 10 , ) x \in (\dfrac{4}{3},5-\sqrt{6}) \cup (10,\infty)

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1 solution

Arjen Vreugdenhil
Feb 18, 2016

Calm down! In general, log b a > 0 { b > 1 and a > 1 or 0 < b < 1 and 0 < a < 1. \log_b a > 0\ \ \Leftrightarrow\ \ \begin{cases} b > 1\ \text{and}\ a > 1 & \text{or} \\ 0 < b < 1\ \text{and}\ 0 < a < 1. & \end{cases}

We must therefore determine the set ( A C ) ( B D ) (A \cap C) \cup (B \cap D) , with the sets A , B , C , D A, B, C, D defined as the solution sets: ( A ) : x 2 12 x + 30 10 > 1 ( B ) : 0 < x 2 12 x + 30 10 < 1 ( C ) : log 2 2 x 5 > 1 2 x 5 > 2 ( D ) : 0 < log 2 2 x 5 < 1 1 < 2 x 5 < 2 (A):\ \frac{x^2 - 12x + 30}{10} > 1 \\ (B):\ 0 < \frac{x^2 - 12x + 30}{10} < 1 \\ (C):\ \log_2 \frac{2x}{5} > 1\ \ \therefore\ \ \frac{2x}5 > 2 \\ (D):\ 0 < \log_2 \frac{2x}{5} < 1\ \ \therefore\ \ 1 < \frac{2x}5 < 2

The solutions sets are A = , 2 10 , ; B = 2 , 6 6 6 + 6 , 10 ; C = 5 , ; D = 5 2 , 5 . A = \langle \leftarrow, 2 \rangle \cup \langle 10, \rightarrow \rangle; \\ B = \langle 2, 6 - \sqrt 6 \rangle \cup \langle 6 + \sqrt 6, 10 \rangle; \\ C = \langle 5, \rightarrow \rangle; \\ D = \langle \tfrac52, 5 \rangle.

The solution set of the inequality is therefore ( A C ) ( B D ) = 10 , 5 2 , 6 6 . (A \cap C) \cup (B \cap D) = \langle 10, \rightarrow \rangle \cup \langle \tfrac52, 6-\sqrt 6\rangle.

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