f(-x)=-f(x)
f(-x)=log4[-x+√(x^2+a/4)]
=log4{[√(x^2+a/4)-x][√(x^2+a/4)+x]/[√(x^2+a/4)+x]}
=log4 {a/4[√(x^2+a/4)+x]}
=log4 a-{1+log4[√(x^2+a/4)+x]}
=log4a-1-f(x)=log4a-1+f(-x)
所以log4a-1=0,a=4
log4 (a+4)=log4 8=log4 4^(3/2)=3/2