Cho tam giác ∆ABC chứng minh rằng: a) \(\sin \left( {\frac{{\widehat A + \widehat B}}{2}} \right) = \cos \frac{{\widehat C}}{2}\); b) \(\tan \left( {2\widehat A + \widehat B + \widehat C} \right) = \tan \widehat A\); c) \(\sin \left( {\frac{{\widehat A + \widehat B + 3\widehat C}}{2}} \right) = \cos \widehat C\).

Cho tam giác ∆ABC chứng minh rằng:

a) \(\sin \left( {\frac{{\widehat A + \widehat B}}{2}} \right) = \cos \frac{{\widehat C}}{2}\);

b) \(\tan \left( {2\widehat A + \widehat B + \widehat C} \right) = \tan \widehat A\);

c) \(\sin \left( {\frac{{\widehat A + \widehat B + 3\widehat C}}{2}} \right) = \cos \widehat C\).