Undefined control sequence \bra

[nicolas-tesla-x]

I got an error: Undefined control sequence \bra,

Here's my formula code：

\[\bra{E,\alpha}W_{q}\ket{E,\beta} =\bra{E,\alpha}U_{g_{1}}^{\dagger}W_{q}U_{g_{1}}\ket{E,\beta}=e^{i( \beta-\alpha)}\bra{E,\alpha}W_{q}\ket{E,\beta},\]
\[\Rightarrow\quad\bra{E,\alpha}W_{q}\ket{E,\beta} =0. \tag{3.8}\]

When I use MathJax, it renders normally：

I added a package here and it works：