(2x² + ax - y + 6)—(2bx² + 3x + 5y - 1)
=(2-2b)x²+(a-3)x-6y+7的值与x的取值无关
2-2b=0
a-3=0
b=1,a=3
a²/2-2b²+4ab
=9/2-2+12
=14.5