（1）（8a-7b)-2(4a-5b)
=8a-7b-8a+10b
=(8a-8a)+(10b-7b)
=3b
(2)3(7㎡n-5mn)-4(4n㎡-5mn)
=21㎡n-15mn-16n㎡+20mn
=(21㎡n-16㎡n)+(20mn-15mn)
=5㎡n+5mn
(3)3a²-4a²-2a+2a²-6a
=a²-8a
当a=2时
上式=2²-8×2
=4-16
=-12
（4）2（x-2y）-4（x-2y）+（x-2y）-3(x-2y)
=(x-2y)(2-4+1-3)
=(-4)×(x-2y)
=-4x+8y
当x=-1,y=1／2时
上式=-4×(-1)+8×1／2
=4+4=8
(5)8m²-﹛4m²-2m-(2m²-5m)﹜
=8m²-﹛4m²-2m-2m²+5m﹜
=8m²-4m²+2m+2m²-5m
=6m²-3m
(6)3a²+(4a²-2a-1)-2(3a²-a+1)
=3a²+4a²-2a-1-6a²+2a-2
=a²-3
当a=1／2时
上式=(1／2)²-3
= 1／4-3
=-2又3／4