4/(a²-1)-2/(a²+a)
问题描述：
4/(a²-1)-2/(a²+a)
(1/x³-1/x²+1/x)*x³
答
4/(a^2-1)-2/(a^2+a) =4/(a-1)(a+1)-2/a(a+1) =[4a-2(a-1)]/[a(a-1)(a+1)]=(4a-2a+2)/[a(a-1)(a+1)]=(2a+2)/[a(a-1)(a+1)]=2(a+1)/[a(a-1)(a+1)]=2/[a(a-1)](1/x^3-1/x^2+1/x)*x^3=1-x+x^2