已知1/a+1/b=4,求（4a+3ab+4b)/(-3a+2ab-3b)的值
问题描述：
已知1/a+1/b=4,求（4a+3ab+4b)/(-3a+2ab-3b)的值
答
1/a+1/b=4
(a+b)/ab=4
a+b=4ab
（4a+3ab+4b)/(-3a+2ab-3b)
=[4(a+b)+3ab]/[-3(a+b)+2ab]
=(16ab+3ab)/(-12ab+2ab)
=19ab/(-10ab)
=-19/10