a,b,c成等比数列且f(0) = -4
可以设b=cm,a=cm^2
所以
f(0)=c=-4
b=-4m
a=-4m^2
f(x)=-4m^2x^2-4mx-4=-(2mx-1)^2-3
则当x=1/2m时
f(x)有最大值,为-3