由f^2(x+1)+f^2(x)=9知f^2(x+2)+f^2(x+1)=9,f(x)>=0得到f(x+2)=f(x),函数周期为2,
f(2013/6)=f(3/2) f^2(3/2)+f^2(1/2)=9,f(1/2)=2,求得f(2013/6)=f(3/2) =根号5