（1）原式=2xy+x(x−y)+y(x+y)x2−y2=(x+y)2(x+y)(x−y)=x+yx−y；（2）原式=2a−(a+2)(a+2)(a−2)a−2(a+2)(a−2)=1a+2；（3）原式=x2+2xx−1•x−1x=x(x+2)x−1•x−1x=x+2；（4）原式=2(a+1)a−1•1a+1-(a+1)(a−...