Question

cosа+cos2а+cos3а=? если а=7п/6​

Answer 1

$Cos\alpha+Cos2\alpha+Cos3\alpha=Cos\frac{7\pi }{6}+Cos2*\frac{7\pi }{6}+Cos3*\frac{7\pi }{6}=Cos(\pi+\frac{\pi }{6})+Cos\frac{7\pi }{3}+Cos\frac{7\pi }{2}=-Cos\frac{\pi }{6}+Cos(2\pi+\frac{\pi }{3})+Cos(2\pi+\frac{3\pi }{2})=-Cos\frac{\pi }{6}+Cos\frac{\pi }{3}+Cos\frac{3\pi }{2}=-\frac{\sqrt{3}}{2}+\frac{1}{2}+0=\frac{1-\sqrt{3}}{2}$