2A+4B
=2(2x平方+3kx-2x-1)+4(-x平方+kx-1)
=4x²+6kx-4x-2-4x²+4kx-4
=(10k-4)x-6
与x的值无关
∴10k-4=0
k=0.4
k³=0.4³=0.064