f(1)=3*1^2-4=-1
f(2)=3*2^2-4=8>0 => f[f(2)]=f(8)=3*8^2-4=188
a>-1时,f(a+1)=3(a+1)^2-4=3a^2+6a+2
a=-1时,f(a+1)=f(0)=pai
a