2a的2次方b-[2ab的2次方-2(a的2次方b+2ab的2次方)]
=2a^2b-2ab^2+2a^2b+4ab^2
=4a^2b+2ab^2
=2ab(2a+b)
=2ab(2×（-1/2）+1)
=2ab×0
=0