原式=[2x/(x+y)+4x/(x-y)]/8x*(x+y)(x-y)
=2x[(x-y+2x+2y)/(x+y)(x-y)]/8x*(x+y)(x-y)
=(3x+y)/(x+y)(x-y)4*(x+y)(x-y)
=(3x+y)/4