若代数式（2x²+ax-y+6)-(2bx²-3x+5y-1)的值与字母x无关,
（2x²+ax-y+6)-(2bx²-3x+5y-1)
=(2-2b)x²+(a+3)x-6y+7
=7-6y,[代数式的值与字母x无关,代数式≡7-6y]
2-2b=0,b=1;
a+3=0,a=-3
(1/2)a²-2b²+4ab
=(1/2)(9)-2+4(-3)(1)
=9/2-2-12
=-19/2