Giải thích các bước giải:
Ta có:
\(\begin{array}{l}
\widehat A + \widehat B + \widehat C = 180^\circ \Rightarrow \widehat A + \widehat B = 180^\circ - \widehat C\\
a,\\
\sin x = \sin \left( {180^\circ - x} \right)\\
\Rightarrow \sin \left( {\widehat A + \widehat B} \right) = \sin \left( {180^\circ - \left( {\widehat A + \widehat B} \right)} \right) = \sin \widehat C\\
b,\\
\cos x = - \cos \left( {180^\circ - x} \right)\\
\cos \left( {\widehat A + \widehat B} \right) = - \cos \left( {180^\circ - \left( {\widehat A + \widehat B} \right)} \right) = - \cos \widehat C\\
c,\\
\sin x = \cos \left( {90^\circ - x} \right)\\
\sin \frac{{A + B}}{2} = \cos \left( {90^\circ - \frac{{A + B}}{2}} \right) = \cos \frac{{180^\circ - \widehat A - \widehat B}}{2} = \cos \frac{C}{2}
\end{array}\)
