Đáp án + Giải thích các bước giải:

$\textbf{b$\bigg)$} VT = \sin \dfrac{B + C}{2}$ 

$= \sin \dfrac{180^\circ - A}{2}$

$= \sin \bigg(90^\circ - \dfrac{A}{2}\bigg)$

$= \cos \dfrac{A}{2} = VP$

$\textbf{d$\bigg)$} VT = \cos (A + B - C)$

$= \cos (180^\circ - C - C)$

$= \cos (180^\circ - 2C)$

$= -\cos 2C = VP$

$\textbf{f$\bigg)$} VT = \tan \dfrac{A + B - 2C}{2}$

$= \tan \dfrac{180^\circ - C -2 C}{2}$

$= \tan \dfrac{180^\circ - 3C}{2}$

$= \tan \bigg(90^\circ - \dfrac{3C}{2}\bigg)$

$= \cot \dfrac{3C}{2} = VP$

$\text{_______________________}$

Công thức được sử dụng:

$\sin (90^\circ - \alpha) = \cos \alpha$

$\cos (180^\circ - \alpha) = -\cos \alpha$

$\tan (90^\circ - \alpha) = \cot \alpha$