已知函数 $f\left(x\right)=\sin x+\tan x$.项数为27的等差数列 $\left\{a_n\right\}$满足 $a_n\in \left(-\frac{\pi }{2},\frac{\pi }{2}\right)$，且公差 $d\ne 0$.若 $f\left(a_1\right)+f\left(a_2\right)+\dots +f\left(a_{21}\right)=0$，则当 $k$=    时， $f\left(a_k\right)=0$.