(x-y)/(2x+2y) =(x-y)/2(x+y)=(x-y)(x+y)/2(x+y)²=(x²-y²)/2(x+y)²xy/(x+y)²=2xy/2(x+y)²x-y/2x+2y 与 xy/(x+y)²相加= (x²-y²)/2(x+y)²+2xy/2(x+y)²=(x&sup...