What is \(\mathop \smallint \limits_{0{\rm{\;}}}^{\frac{{\rm{\pi }}}{2}} \frac{{{\rm{dx}}}}{{{{\rm{a}}^2}{{\cos }^2}{\rm{x}} + {{\rm{b}}^2}{{\sin }^2}{\rm{x}}}}\) equal to?
1
2ab
2
2πab
3
\(\frac{{\rm{\pi }}}{{2{\rm{ab}}}}\)
4
\(\frac{{\rm{\pi }}}{{{\rm{ab}}}}\)