Cho sina=-12/13;3π/2<a<2π Tính cos (π/3-a) Giải hộ mình

$\frac{3\pi}{2}<a<2\pi$

$\Rightarrow cosa>0$

$\Rightarrow cosa=\sqrt{1-sin^2a}=\frac{5}{13}$ 

$cos(\frac{\pi}{3}-a)= cos\frac{\pi}{3}.cosa+sin\frac{\pi}{3}.sina$

$= \frac{1}{2}.\frac{5}{13}+\frac{\sqrt3}{2}.\frac{-12}{13}$

$= \frac{5-12\sqrt3}{26}$