Đáp án:
$\begin{array}{l}
1)\sin {27^0} - \cos {63^0}\\
= \cos \left( {{{90}^0} - {{27}^0}} \right) - \cos {63^0}\\
= \cos {63^0} - \cos {63^0}\\
= 0\\
2)\\
\dfrac{{\tan {{31}^0}.\tan {{59}^0}}}{{\cot {{16}^0}.\cot {{74}^0}}} = \dfrac{{\tan {{31}^0}.\cot {{31}^0}}}{{\cot {{16}^0}.\tan {{16}^0}}} = \dfrac{1}{1} = 1\\
3)\\
\sin {15^0} + \sin {20^0} - \cos {70^0} - \cos {75^0}\\
= \cos {75^0} + \cos {70^0} - \cos {70^0} - \cos {75^0}\\
= 0
\end{array}$