x^3+2x^2y-8xy^2 取x=2 y=1时,等于多少
x^3+2x^2y-8xy^2
=x(x^2+2xy-8y^2)
=x(x+4y)(x-2y)
x=2 y=1时
原式=2*(2+4)(2-1)
=12