求20道初二的化简求值题急用

1．(3a2-4ab-5b2)-(2b2-5a2+2ab)-(-6ab)．
2．xy-(2xy-3z)+(3xy-4z)．
3．(-3x3+2x2-5x+1)-(5-6x-x2+x3)．
4．3x-(2x-4y-6x)+3(-2z+2y)．
5．(-x2+4+3x4-x3)-(x2+2x-x4-5)．
6．若A=5a2-2ab+3b2,B=-2b2+3ab-a2,计算A+B．
7．5m2n+(-2m2n)+2mn2-(+m2n)．
8．2(x2-2xy+y2-3)+(-x2+y2)-(x2+2xy+y2)．
9．2(a2-ab-b2)-3(4a-2b)+2(7a2-4ab+b2)．
10．求(3x2y-2xy2)-(xy2-2x2y),其中x=-1,y=2．
11．已知|x+1|+(y-2)2=0,求代数式5(2x-y)-3(x-4y)的值．
12．当P=a2+2ab+b2,Q=a2-2ab-b2时,求P-[Q-2P-(P-Q)]．
13．求2x2-｛-3x+5+[4x2-(3x2-x-1)]｝的值,其中x=-3．
14．当x=-2,y=-1,z=3时,求5xyz-｛2x2y-[3xyz-(4xy2-x2y)]｝的值．
15．已知A=x3-5x2,B=x2-6x+3,求A-3(-2B)．
16．去括号：｛-[-(a+b)]｝-｛-[-(a-b)]｝．
17．去括号：-[-(-x)-y]-[+(-y)-(+x)]．
18．合并同类项：5m2n+5mn2-mn+3m2n-6mn2-8mn．