1】2a-(5a-3b)+(7a-b)
=2a-5a+3b+7a-b
=2a+7a-5a+3b-b
=4a+2b
【2】3x²y-[2x²y-(2xyz-x²z)-4x²z]
=3x²y-(2x²y-2xyz+x²z-4x²z)
=3x²y-2x²y+2xyz-x²z+4x²z
=x²y+2xyz+3x²z
[3] 2.8x²-[-3x+(2x²-3x)+3]
=2.8x²-(-3x+2x²-3x+3)
=2.8x²+6x-2x²-3
=0.8x²+6x-3
[4]4a-(2a-b)-7(a+2b)
=4a-2a+b-7a-14b
=-5a-13b
[5]3a²b-[a²c-(3abc+2a²c)+5a²b]-3abc
=3a²b-(a²c-3abc-2a²c+5a²b)-3abc
=3a²b-a²c+3abc+2a²c-5a²b-3abc
=-2a²b+a²c
[6]先化简,后求值：5(3a²b-ab²)-(ab²+3a²b),其中2=1/2【二分之一】,b=-1
5(3a²b-ab²)-(ab²+3a²b)
=15a²b-5ab²-ab²-3a²b
=12a²b-6ab²
=6ab(2a-b)
=6*(1/2)*(-1)*(2*(1/2)-(-1))
=-3*2
=-6
[7]先化简,后求值:1/2x-2(x-1/3y²)+(-3/2x+1/3y²),其中x=-2,y=-1
1/2x-2(x-1/3y²)+(-3/2x+1/3y²)
=(x/2)-2x+(2y²/3)-(3x/2)+(y²/3)
=-3x+y²
=-3*(-2)+(-1）²
=6+1
=7