1/a+1/b=3
通分可得
(a+b)/(ab)=3
a+b=3ab
(2a-3ab+2b)/(9a+7ab+9b)
=[2(a+b)-3ab]/[9(a+b)+7ab]
=(2×3ab-3ab)/(9×3ab+7ab)
=(6ab-3ab)/(27ab+7ab)
=(3ab)/(34ab)
=3/34