a的四次方 +3a的三次方 +3a的平方 +2a =
=a(a³+3a²+3a+2)
=a(a³+2a²+a²+2a+a+2)
=a[a²(a+2)+a(a+2)+(a+2)]
=a(a+2)(a²+a+1)