先化简再求值,计算好点的进

题目

先化简再求值,计算好点的进
3x^y-[2x^2y-(2xy-1/2 x^2)-4x^2]-xy,其中x=2,y=1/3.
(7a^2n -2b^2m+1 +c)-(-2a^2n +4b^2m+1 +c),其中a=1,b=-1,m,n都为正整数.
已知:(a+3)^2 +|b+1|+c^2=0,M=3a^2 b -2ab^2 c-4ab^3,N=3/2 ab^2 c -a^2 b +2ab^3.求M-N的值
忘,

答案

3x^y-[2x^2y-(2xy-1/2 x^2)-4x^2]-xy=3x^y-(2x^2y-2xy+1/2 x^2-4x^2)-xy=3x^y-(2x^2y-2xy-7/2 x^2)-xy=3x^y-2x^2y+2xy+7/2 x^2-xy=x^2y+xy+7/2 x^2=2^2*(1/3)+2*(1/3)+(7/2)*2^2=4/3+2/3+14=16(7a^2n -2b^2m+1 +c)-(...