1.先化简再求值,3a^2-3ab/a^2-2ab+b^2,a=8,b=7

题目

1.先化简再求值,3a^2-3ab/a^2-2ab+b^2,a=8,b=7
2.若(1/x)-(1/y)=2,求2x3xy-2y/x-2xy-y

答案

3a^2-3ab/a^2-2ab+b^2
=3a(a-b)/(a-b)²
=3a/(a-b)
当a=8,b=7时
原式=3×8/(8-7)
=24
(2x+3xy-2y)/(x-2xy-y)
=(2/y+3-2/x)/(1/y -2-1/x) 分子分母同时除以xy
=[-2(1/x-1/y)+3]/[-1/x-1/y)-2]
=(-4+3)/(-2-2)
=1/4