x/3=y/4,求(x*x-xy)/(x*x+y*y)的值
问题描述：
x/3=y/4,求(x*x-xy)/(x*x+y*y)的值
答
令x/3=y/4=k
x=3k,y=4k
(x^2-xy)/(x^2+y^2)
=(9k^2-12k^2)/(9k^2-16k^2)
=-3k^2/(-7k^2)
=3/7