Найдите cos a, если tg a..

-1/корень из 10

tg a = sin a / cos a = 3 =>sin a = 3*cos asin^2 a + cos^2 a = 13cos^2 a + cos^2 a = 1cos^2 a = 1/4cos a = + - 1/2pi/2 < a < pi -----> сos a = - 1/2