问题 解答题

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.

答案

(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.

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