数学作业求助～～～（初一）
1．已知a^3+b^3=27,a^2b-ab^2=-6,求代数式（b^3-a^3)+(a^2b-3ab^2)-2(b^3-ab^2)的值
2．已知X－Y＝5，M－N＋Y＝40，则M＾2－25的值是＿＿＿．
3．若X+2Y+3Z=10,4X+3Y+2Z=15,则1/2(X+Y+Z)=_______.
4.已知-X+3y=5,则5(x-3y)^2-8(x-3y)-5的值是( ).
(能说原因）谢谢
参考答案:1．已知a^3+b^3=27,a^2b-ab^2=-6,
代数式（b^3-a^3)+(a^2b-3ab^2)-2(b^3-ab^2)
=b^3-a^3+a^2b-3ab^2-2b^3+2ab^2
=-a^3-b^3+a^2b-ab^2
=-(a^3+b^3)+(a^2b-ab^2)
=-27+(-6)
=-33
2．已知X－Y＝5，M－X＋Y＝40，
M-(X-Y)=40
M=45
则M＾2－25的值是45^2-25=2000.
3．若X+2Y+3Z=10,4X+3Y+2Z=15,
5X+5Y+5Z=25
X+Y+Z=5
则1/2(X+Y+Z)=5/2
4.已知-X+3y=5,则
X-3y=-5
5(x-3y)^2-8(x-3y)-5
=5*(-5)^2-8*(-5)-5
=125+40-5
=160