how to configure it to support rendering `\( a + b \)` as formula rather than plaintext?

[redmouth]

how to configure it to support rendering \( a + b \) as formula rather than plaintext?

With following configuration

let g:mkdp_auto_start = 1
let g:mkdp_preview_options = {
\ 'mathjax': {
\   'inlineMath': [ ['$', '$'], ['\\(','\\)'] ],
\   'displayMath': [ ['\\[','\\]'] ],
\ }
\ }

\( a + b \) is not treated as math formula