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Algebra Difficulty 5.5 AIME, harder Find the answer

Find the set of solutions for xx in the inequality x+1x+2>3x+42x+9\frac{x+1}{x+2} > \frac{3x+4}{2x+9} when x2,x92x \neq -2, x \neq \frac{9}{2}.

A number or a short expression. Spacing and $ signs are ignored.

Solution

There are 3 possible cases of xx: 1) 92<x-\frac{9}{2} < x, 2) 92x2\frac{9}{2} \leq x \leq -2, 3) 2<x-2 < x. For the cases (1) and (3), x+2x+2 and 2x+92x+9 are both positive or negative, so the following operation can be carried out without changing the inequality sign: x+1x+2>3x+42x+92x2+11x+9>3x2+10x+80>x2x1\begin{aligned} \frac{x+1}{x+2} & > \frac{3x+4}{2x+9} \\ \Rightarrow 2x^{2} + 11x + 9 & > 3x^{2} + 10x + 8 \\ \Rightarrow 0 & > x^{2} - x - 1 \end{aligned} The inequality holds for all 152<x<1+52\frac{1-\sqrt{5}}{2} < x < \frac{1+\sqrt{5}}{2}. The initial conditions were 92<x-\frac{9}{2} < x or 2<x-2 < x. The intersection of these three conditions occurs when 152<x<1+52\frac{1-\sqrt{5}}{2} < x < \frac{1+\sqrt{5}}{2}. Case (2) is 92x2\frac{9}{2} \leq x \leq -2. For all xx satisfying these conditions, x+2<0x+2 < 0 and 2x+9>02x+9 > 0. Then the following operations will change the direction of the inequality: x+1x+2>3x+42x+92x2+11x+9<3x2+10x+80<x2x1\begin{aligned} \frac{x+1}{x+2} & > \frac{3x+4}{2x+9} \\ \Rightarrow 2x^{2} + 11x + 9 & < 3x^{2} + 10x + 8 \\ \Rightarrow 0 & < x^{2} - x - 1 \end{aligned} The inequality holds for all x<152x < \frac{1-\sqrt{5}}{2} and 1+52<x\frac{1+\sqrt{5}}{2} < x. The initial condition was 92x2\frac{-9}{2} \leq x \leq -2. Hence the intersection of these conditions yields all xx such that 92x2\frac{-9}{2} \leq x \leq -2. Then all possible cases of xx are 92x2152<x<1+52\frac{-9}{2} \leq x \leq -2 \cup \frac{1-\sqrt{5}}{2} < x < \frac{1+\sqrt{5}}{2}.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.