计算 （1)1⼀2-a+4⼀a^2-4 (2)1⼀2-x+4⼀x^2-4+x-1⼀2+x

（1)1/2-a+4/a^2-4
=1/(2-a)+4/(a^2-4)
=-1/(a-2)+4/(a+2)(a-2)
=[4-(a+2)]/[(a+2)(a-2)]
=(2-a)/[(a+2)(a-2)]
=-1/(a+2)
(2)1/2-x+4/x^2-4+x-1/2+x
=1/(2-x)+4/(x^-4)+(x-1)/(x+2)
=-1/(x-2)+4/(x-2)(x+2)+(x-1)/(x+2)
=[-(x+2)+4+(x-1)(x-2)]/[(x-2)(x+2)]
=(-x-2+4+x^2-3x+2)/[(x-2)(x+2)]
=(x^2-4x+4)/[(x-2)(x+2)]
=(x-2)^2/[(x-2)(x+2)]
=(x-2)/(x+2)
(3)2x^2/(x-y)^2+x^2-4xy/(y-x)^2-x^2-2y^2/x^2-2xy+y^2
=2x^2/(x-y)^2+(x^2-4xy)/(x-y)^2-(x^2-2y^2)/(x-y)^2
=(2x^2+x^2-4xy-x^2+2y^2)/(x-y)^2
=(2x^2-4xy+2y^2)/(x-y)^2
=2(x-y)^2/(x-y)^2
=2

真的看不懂，整个分母或分子要用（），不然不清楚！