Question

已知1/a+1/b=4,则(a-3ab+b)/(2a+2b+7ab)=

Answer 1

分子分母同时除以ab

=(1/b-3+1/a)/(2/b+2/a-7)
=(4-3)/(8-7)
=1

Answer 2

(a-3ab+b)/(2a+2b+7ab)
=[1/(ab)*(a-3ab+b)]/[1/(ab)*(2a+2b+7ab)] 【分子、分母同乘(ab)分之1】
=(1/b-3+1/a)]/(2/b+2/a+7)
=(4-3)]/[2(1/b+1/a)+7]
=1/[2(4)+7]
=1/15