已知:x/y=4/5,求y²/(x²-xy+y²)的值
问题描述：
已知:x/y=4/5,求y²/(x²-xy+y²)的值
答
解
x/y=4/5
∴x=4/5y
∴
y²/(x²-xy+y²)
=y²/[(4/5y)²-(4/5y)y+y²]
=y²/[16/25y²-4/5y²+y²]
=y²/(-4/25y²+y²)
=y²/(21/25y²)
=25/21