(1+x-2x^2)(x+1/x)^8
=(1+x-2x^2)[ x^8+8x^6+28x^4+56x^2+70+56(1/x^2)+28(1/x^4)+8(1/x^6)+1/x^8 ]
coef.of x^4
=1(28)-2(56)
=-84二项式定理怎么算？？？