Question
if $\left(a^2+b^2\right)\left(x^2+y^2\right)= (ax + by)^2 ;$ prove that $\frac{a}{x}=\frac{b}{y}$

Answer

Given $\left(a^2+b^2\right)\left(x^2+y^2\right)=(a x+b y)^2$
$a^2 x^2+a^2 y^2+b^2 x^2+b^2 y^2+b^2 y^2+2 a b x y $
$a^2 y^2+b^2 x^2-2 a b x y=0 \\ (a y-b x)^2=0 $
$ \text { ay - bx }=0 $
$ \text { ay }=\text { bx } $
$ \frac{a}{x}=\frac{b}{y}$

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