WMC 2018 Pursuit - Pack 1, D3

Algebra Level pending

If a a and b b are positive real numbers with a b a\neq b and x x and y y are solutions to the system below, what is the greatest lower bound of x y \left|\frac{x}{y}\right| ?

{ a x + b y = a b b x + a y = b a \begin{cases} ax+by=\frac{a}{b} \\ bx+ay=\frac{b}{a} \end{cases}

WMC Pursuit Questions


The answer is 3.

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1 solution

Julian Yu
Oct 6, 2018

If we solve the system for x x and y y , we get:

x = a 2 + a b + b 2 a b ( a + b ) x=\dfrac{{{a}^{2}}+ab+{{b}^{2}}}{ab(a+b)}\hspace{0.2cm} and y = 1 a + b \hspace{0.2cm}y=-\dfrac{1}{a+b}

Therefore, x y = a 2 + a b + b 2 a b = 1 + a b + b a \left| \dfrac{x}{y} \right|=\dfrac{{{a}^{2}}+ab+{{b}^{2}}}{ab}=1+\dfrac{a}{b}+\dfrac{b}{a} . But a b + b a 2 \dfrac{a}{b}+\dfrac{b}{a}\geq 2 by AM-GM, so the minimum possible value for x y \left| \dfrac{x}{y} \right| is 1 + 2 = 3 1+2=\boxed{3} .

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