Question

Определите знак выражения

Answer 1

$\displaystyle\bf\\Sin\frac{9\pi }{5} =Sin\Big(2\pi -\frac{\pi }{5}\Big)=-Sin\frac{\pi }{5} <0\\\\\\Ctg\Big(-\frac{8\pi }{7} \Big)=-Ctg\Big(\pi +\frac{\pi }{7} \Big)=-Ctg\frac{\pi }{7} <0\\\\\\\underbrace{Sin\frac{9\pi }{5} }_{<0}\cdot \underbrace{Ctg\Big(-\frac{8\pi }{7} \Big)}_{<0}>0$

Ответ : > 0