A=2a²－a B=－5a+1
3A－2B+1
=3*a（2a-1）-2（－5a+1）+1
=6a²-3a+10a-2+1
=6a²+7a-1
=（a+1）（6a+1）-2
当a=1/2时：
（a+1）*（6a+1）-2
=6-2
=4