先化简,再求值:(x²)/(x²-2xy+y²)÷(x²+2xy+y²)/(x²-y²),其中x=3
问题描述:
先化简,再求值:(x²)/(x²-2xy+y²)÷(x²+2xy+y²)/(x²-y²),其中x=3
y=4
答
(x²)/(x²-2xy+y²)÷(x²+2xy+y²)/(x²-y²)
=(x²)/(x-y)²÷(x+y)²/(x+y)(x-y)
=x² / (x-y)² * (x+y)(x-y) / (x+y)²
=x² / (x-y)(x+y)
=x² / (x²-y²)
把x=3,y=4代入
原式=3² / (3²-4²)
=-9/7