x的四次方-3x的立方-5x的平方+3x+4
=（x^4+x^3）-4(x^3+x^2)-(x^2+x)+4(x+1)
=(x^3-4x^2-x+4)(x+1)
={x^2(x-4)-(x-4)}(x+1)
=(x-4)(x^2-1)(x+1)
=(x-4)(x-1)(x+1)^2