x^3+2x-12
=x^3+2x-4-8
=(2x-4)+(x^3-8)
=2(x-2)+(x-2)(x^2+2x+4)
=(x-2)(x^2+2x+6)
由于：x^2+2x+6恒大于0,
所以x-2=0
即：x=2