1/a+1/b=3则a-ab+b/a+2ab+b值是?

1/a+1/b=3则a-ab+b/a+2ab+b值是?
1/a+1/b=3则a-ab+b/a+2ab+b值是?

1/a+1/b=3则a-ab+b/a+2ab+b值是?
∵1/a+1/b=3
∴a+b=3ab
﹙a-ab+b﹚/﹙a+2ab+b﹚
=﹙3ab-ab﹚／﹙3ab+2ab﹚
=2ab／5ab
=2／5

先告诉你思路，要不以后你遇到这样的题也不会做。把上面式子化成ab=3(a+b)，再替换到求的式中，约分，就出来了 a-ab+b/a+2ab+b
= (1/b - 1 + 1/a) / (1/b + 2 + 1/a)
= [(1/a +1/b) - 1] / [(1/a +1/b) + 2]
= (3 - 1) / (3 + 2)
= 2 / 5