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人教版初中八年级数学分式的运算解答题练习含答案16.先化简,再求值:1x+1-1xx2-1,其中x=2+1. 【答案】解:原式=1x+1-x+1x+1x+1x-1x=-xx+1x+1x-1x=-x-1=1-x,当x=2+1时,原式=1-x=1-2+1=1-2-1=-2.17. (1)先化简再求值: 1-1x+2x2-1x+2,其中x=2;(2)化简计算: xx+y+yx-y-2xyx2-y2.【答案】解:(1)原式=x+2x+2-1x+2x+2(x+1)(x-1)=x+1x+2x+2(x+1)(x-1)=1x-1,当x=2时,原式=12-1=1.(2)原式=x(x-y)(x+y)(x-y)+y(x+y)(x-y)(x+y)-2xy(x+y)(x-y)=(x-y)2(x+y)(x-y)=x-yx+y.18. (1)先化简,再求值: 1-3x+2x2-2x+1x2-4,其中x=12;(2)解方程:x2x-3+53-2x=4.【答案】解:(1)1-3x+2x2-2x+1x2-4=x+2x+2-3x+2x2-4x2-2x+1=x-1x+2x+2x-2x-12=x-2x-1.当x=12时,原式 =12-212-1=3(2) x2x-3+53-2x=4 x-5=42x-3, x=1经检验:x=1是原方程的解