2x²+ax-y+6-2bx²+3x-5y-1
=(2-2b)x²+(a+3)x-6y+5
∵代数式2x²+ax-y+6-2bx²+3x-5y-1与x的取值无关
∴2-2b=0,a+3=0
a=-3,b=1
1/3a的三次方-2b²-1/4a的三次方+3b²
=1/12a³+b²
=1/12×(-27)+1
=-5/4