5x^3+8x^2+12x+15
有理数范围里无法分解.如果是
5x^3-8x^2-12x+15则
=5x^3-5x^2-3x^2+3x-15x+15
=5x^2(x-1)-3x(x-1)-15(x-1)
=(x-1)(5x^2-3x-15)