(3A的平方-2AB-1)-{A的平方-2(AB+B+二分之一)}
=3A的平方-2AB-1-A的平方+2AB+B+1/2
=2A的平方+B-1/2
=2×(-1/2)的平方+1/4-1/2
=1/2+1/4-1/2
=1/4