a Cho sinα= \(\frac{3}{5}\) sinα= \(\frac{3}{5}\) , \(0< \alpha< \frac{\pi}{2}\). Tính \(\sin\left(\alpha+\frac{\pi}{6}\right)\) ,sin 2α
b Cho sinα= \(-\frac{4}{5}\) sinα= \(-\frac{4}{5}\), \(\frac{\pi}{2}< \alpha< \pi\) Tính \(\cos\left(\alpha-\frac{\pi}{3}\right)\),cos 2α