18.原式=(a+1)(a-1)/[a(a+1)]÷[(a-1)^2/a]=(a-1)/a*a/(a-1)^2=1/(a-1)当a=√2+2时原式=1/运郑(√2+1)闷含=√2-1.其他题目看不蚂悄笑清