a+b+c=0
c=-a-b
(-2b/3a)^2-4c/3a
=4b^2/9a^2+4(a+b)/3a
=4b^2/9a^2+4b/3a+4
令m=b/a
则=(4/9)m^2+(4/3)m+4
=(4/9)(m^2+3m)+4
=(4/9)(m^2+2*m*3/2+9/4-9/4)+4
=(4/9)(m+3/2)^2-(4/9)(9/4)+4
=(4/9)^2(m+3/2)^2+3
=(4/9)(b/a+3/2)^2+3
和你所给的答案不一样