19.7/20
20.x≤1且x≠-1
21.(n+1)/2n
22.3/4
23.1
24.1
25.4/45
26.1999/2002
27.a=-1,b=3
28 4
29.m=6
题目还是好多的,答完了应该都是对的.不明白追问下.