(2a²b-5ab)-2(-ab-a²b)=2a²b-5ab+2ab+2a²b=4a²b-3ab
1-3（x-1/2y²）+(-x+1/2y²）=1-3x+3/2y²-x+1/2y²=1-4x+2y²
（-x²y+3xy-4)+3(x²y-xy+2)=-x²y+3xy-4+3x²y-3xy+6=2x²y+2
-1/4（2k³+4k²-28）+1/2（k³-2k²+4k)=-1/2k³-k²+7+1/2k³-k²+2k=-2k²+2k+7
3x²-（2x²+5x-1）-（3x+1)=x²-5x+1-1-3x=x²-8x ,其中x=10,所以結(jié)果=100-80=20


（xy-3/2y-1/2)-(xy-3/2x+1)=xy-3/2y-1/2-xy+3/2x-1=3/2x-3/2y-3/2 ,其中x=10/3  y=8/3


所以結(jié)果=3/2(x-y-1)=3/2(10/3-8/3-1)=3/2(-1/3)=-1/2