A＝2a²-a,B=-5a+1 可得：
3A-2B+1
=3(2a²-a)-2(-5a+1)+1
=6a²-3a+10a-2+1
=6a²+7a-1
当a=1/2时,
原式=6x(1/2)²+7x1/2-1
=3/2+7/2-1
=4