1/(x²-y²)+ 2 /（x-y）²?
=1/[(x+y)(x-y)]+ 2 /(x-y)²
=(x-y)/[(x+y)(x-y)²]+ 2(x+y)/[(x+y)(x-y)²]
=(x-y+2x+2y)/[(x+y)(x-y)²]
=(3x+y)/[(x+y)(x-y)²]