x²+9=(x+3i)(x-3i)
=(9+x²)(9-x²)=(x+3i)(x-3i)(3+x)(3-x)
=(a-b)²+c²=(a-b+ci)(a-b-ci)
=(x+1)²+2=(x+1+√2i)(x+1-√2i)
factor() ; 。
