已知x^2a=4,
x^a=2
(2x^3a)^2-6x^4a+1(-5x^4a-1)的值
=(4x^6a)-6x^4a*x (-5x^4a*x^(-1))
=4*2^6-6*2^4 (-5*2^4)
=4*64-6*16* (-5*16)
=256+7680
=7936