X^4-Y^4/X^2+2XY+Y^2 乘 Y+X/X^2+Y^2
=[(X^2+Y^2)(X+Y)(X-Y)/(X+Y)^2]*[(X+Y)/(X^2+Y^2]
=(X^2+Y^2)(X+Y)(X-Y)*(X+Y)/[(X+Y)^2(X^2+Y^2]
=X-Y