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Algebra Difficulty 5.8 AIME, harder Prove it Ukraine

Solve the system of equations:
{92(x+y)=1x+1y,x22=3y2. \begin{cases} \frac{9}{2(x+y)} = \frac{1}{x} + \frac{1}{y}, \\ \sqrt{x^2 - 2} = \sqrt{3 - y^2}. \end{cases}

Solution

Consider the first equations of the system. It is easy to see that xy0xy \neq 0, and we
get 92(x+y)=x+yxy9xy=2x2+4xy+2y22y25xy+2x2=0 \text{get } \frac{9}{2(x+y)} = \frac{x+y}{xy} \Leftrightarrow 9xy = 2x^2 + 4xy + 2y^2 \Leftrightarrow 2y^2 - 5xy + 2x^2 = 0

y=5x±25x216x24=5x±3x4y = \frac{5x \pm \sqrt{25x^2 - 16x^2}}{4} = \frac{5x \pm 3x}{4}. It follows that all couples (x,2x)(x, 2x) and (x,12x)(x, \frac{1}{2}x) without (0,0)(0,0) are solutions of the first equation. We are putting these solutions in the second equation. From this equation we have x2+y2=5x^2 + y^2 = 5, x2|x| \ge \sqrt{2} and y3y \le \sqrt{3}. Further, putting (x,2x)(x, 2x) the first condition becomes 5x2=5x=±15x^2 = 5 \Leftrightarrow x = \pm 1. And we have solutions (1,2)(1,2) and (1,2)(-1,-2) but these couples don't satisfy the condition x2|x| \ge \sqrt{2}. Analogously putting (x,12x)(x, \frac{1}{2}x) the first condition becomes x2+14x2=5x2=4x^2 + \frac{1}{4}x^2 = 5 \Leftrightarrow x^2 = 4. Here we have solutions (2,1)(2,1) and (2,1)(-2,-1) which satisfy our conditions.

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