Question

一个多项式加上-2x的立方+4x的平方y+5y的立方后得x的立方-3x的平方y+3y立方,求这个多项式

并求x=-1/2y=1/2时这个多项式的值

Answer 1

楼上都算错了

x³-3x²y+3y³-(-2x³+4x²y+5y³)
=x³-3x²y+3y³+2x³-4x²y-5y³
=3x³-7x²y-2y³

这个多项式的值
3x³-7x²y-2y
=3x(-1/2)³-7x(-1/2)²x1/2-2x(1/2)³
=-3/8-7/8-1/4
=-3/2

追问

确定的吗

追答

确定

Answer 2

这个多项式是x³-3x²y+3y³+2x³-4x²y-5y³

=3x³-7x²y-2y³

x=-1/2,y=1/2
原式=3/4-7/8-1/4=-3/8

Answer 3

(x³-3x²y+3y³)-(-2x³+4x²y+5y³)
=x³-3x²y+3y³+2x³-4x²y-5y³
=3x³-7x²y-2y³
=3*(-1/2)³-7*(-1/2)²*(1/2)-2*(1/2)³
=-3/8-7/8-2/8
=-12/8
=-3/2

Answer 4

？