Đáp án:
$\begin{array}{l}
A = \dfrac{{3\cot {{77}^0}}}{{2\tan {{13}^0}}}\\
- \dfrac{{{{\cos }^2}{{26}^0} + {{\cos }^2}{{64}^0} - {{\cos }^2}{{71}^0} - {{\cos }^2}{{19}^0}}}{{{{\sin }^2}{{34}^0} + {{\sin }^2}{{56}^0} + {{\sin }^2}{{15}^0} + {{\sin }^2}{{75}^0}}}\\
= \dfrac{{3\tan {{13}^0}}}{{2\tan {{13}^0}}}\\
- \dfrac{{{{\cos }^2}{{26}^0} + {{\sin }^2}{{26}^0} - \left( {{{\cos }^2}{{71}^0} + {{\sin }^2}{{71}^0}} \right)}}{{{{\sin }^2}{{34}^0} + {{\cos }^2}{{34}^0} + {{\sin }^2}{{15}^0} + {{\cos }^2}{{15}^0}}}\\
= \dfrac{3}{2} - \dfrac{{1 - 1}}{{1 + 1}}\\
= \dfrac{3}{2}\\
\left( {Do:\left\{ \begin{array}{l}
\tan x = \cot \left( {{{90}^0} - x} \right)\\
\cos x = \sin \left( {{{90}^0} - x} \right)
\end{array} \right.} \right)
\end{array}$
