（1）x²/x-2 - 4x/x-2 + 4/x-2=(x²-4x+4)/(x-2)=(x-2)²/(x-2)=x-2
(2)1/1+x + 2x/1-x²=(1-x+2x)/[(1-x)(1+x)]=(1+x)/[(1-x)(1+x)]=1/(1-x)
(3)2a-(a-1)+a²-1/a+1=a+1+(a+1)(a-1)]/(a+1)=a+1+(a-1)=2a
(4)x²-y²/x+y - 4x(x-y)+y²/2x-y
改为：(x²-y²)/(x+y) - {4x(x-y)+y²}/(2x-y)
=(x+y)(x-y)/(x+y) - {4x²-4xy+y²}/（2x-y）
=(x-y)-{2x-y}²/(2x-y)
=(x-y)-{2x-y}
=x-y-2x+y
=-x