先乘以(x+1)后与(x-1)构成平方差公式以此类推后再除以(x+1)
(x的4次方+1)(x的2次方+1)(x-1)=(x^4+1)(x^2+1)(x-1)=(x^6+x^4+x^2+1)(x-1)再相乘
x7-x6+x5-x4+x3-x2+x-1
