1.(2a^2b)*(-1/2ab^2)^3
=2a²b*(-1/8a^3b^6)
=-1/4a^5b^7
2.（1+m+n）(1+m-n)
=(1+m)²-n²
=1+2m+m²-n²
3.化简求值［（x+2y）^2-(x+y)(3x-y)-5y^2］/2,其中x=-2y=1/2
=(x²+4xy+4y²-3x²-2xy+y²-5y²)/2
=(-2x²+2xy)/2
=-x²+xy 其中x=-2y=1/2
=-(-2)²+(-2)*1/2
=-4-1
=-5