已知M=-xy2+3x2y-1,N=4x2y+2xy2-x (1)化简:4M-3N; (2)当x=-2,y=1时,求4M-3N的值.

问题描述:

已知M=-xy2+3x2y-1,N=4x2y+2xy2-x
(1)化简:4M-3N;
(2)当x=-2,y=1时,求4M-3N的值.

(1)4M-3N=4(-xy2+3x2y-1)-3(4x2y+2xy2-x)
=-4xy2+12x2y-4-12x2y-6xy2+3x
=-10xy2+3x-4;
(2)当x=-2,y=1时,
4M-3N=-10×(-2)×1+3×(-2)-4
=20-6-4
=10.