(x+1)^3=a3x^3+ax^2+a1x+a0
(x+1)(x+1)^2=a3x^3+ax^2+a1x+a0
x^3+3x^2+3x+1=a3x^3+a2x^2+a1x+a0
对比系数：
a3=1,a2=3,a1=3,a0=1