2x^3-8x^2y+2+5x^3+8x^2y-7x^3
=(2+5-7)x³+(-8+8)x²y+2
=2
∴代数式2x^3-8x^2y+2+5x^3+8x^2y-7x^3的值与字母x、y的取值无关