(m+n)^2-(m-n)^2
=m^2+2mn+n^2-(m^2-2mn+n^2)
=2mn+2mn
=4mn
（（x^2-2x）/x-2〕÷x+（x^2-1）/（x^2-2x+1）
=(x-2-2)÷x+(x-1)(x+1)/(x-1)^2
=(x-4)÷x+（x+1)/(x-1)
因为x＝1/2,带入公式得：
=2(1/2-4)+(1/2+1)/(1/2-1)
=(1-8)+(3/2）/(-1/2)
=-7-3
=-10
若X-2是有括号的：
[（x^2-2x）/（x-2〕]÷x+（x^2-1）/（x^2-2x+1）
=[x(x-2)/(x-2)]÷x+(x-1)(x+1)/(x-1)^2
=1+（x+1)/(x-1)
因为x＝1/2,带入公式得：
=1+(1/2+1)/(1/2-1)
=1+(3/2）/(-1/2)
=1-3
=-2