How to use do_shortcode_tag to modify the output of a shortcode?

You have a LaTex plugin? It probably filters “the_content”. Try

return apply_filters( 'the_content', '$\frac{15}{5} = 3$' );

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)