Giải thích các bước giải:
Ta có:
$B=\dfrac{\sin2a-2\sin a}{\sin2a+2\sin a}$
$\to B=\dfrac{2\sin a\cos a-2\sin a}{2\sin a\cos a+2\sin a}$
$\to B=\dfrac{2\sin a(\cos a-1)}{2\sin a(\cos a+1)}$
$\to B=\dfrac{\cos a-1}{\cos a+1}$
Ta có:
$C=\dfrac{\cos^3a-\cos3a}{\cos a}+\dfrac{\sin^3a+\sin3a}{\sin a}$
$\to C=\dfrac{\cos^3a-(4\cos^3a-3\cos a)}{\cos a}+\dfrac{\sin^3a+3\sin a-4\sin^3a}{\sin a}$
$\to C=\dfrac{\cos^3a-4\cos^3a+3\cos a}{\cos a}+\dfrac{\sin^3a+3\sin a-4\sin^3a}{\sin a}$
$\to C=\dfrac{-3\cos^3a+3\cos a}{\cos a}+\dfrac{3\sin a-3\sin^3a}{\sin a}$
$\to C=-3\cos^2a+3+3-3\sin^2a$
$\to C=-3(\sin^2a+\cos^2a)+6$
$\to C=-3\cdot 1+6$
$\to C=3$
