怎么因式分解:(y+1)(y-1)(y+3)(y+5)+15

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怎么因式分解:(y+1)(y-1)(y+3)(y+5)+15

(y+1)(y-1)(y+3)(y+5)+15
=(y-1)(y+5)(y+1)(y+3)+15
=(y^2+4y-5)(y^2+4y+3)+15 把y^2+4y看做一个整体
=(y^2+4y)^2-2(y^2+4y)-15+15
=(y^2+4y)(y^2+4y-2)
=y(y+4)(y^2+4y-2)
有问题追问.