17, a) = cos²t + cos²t =2cos²t; б) [tex] \frac{ - cost· tgt }{- sint} = \frac{- cot \frac{sint}{cost} }{- sint}
= \frac{- sint}{- sint} =1;
20. a) = π + π/3 = 4π/3; б) π/3 + π/6 = π/2; b) 3π/4 - π/6 = 7π/12;
г) π/2 + π/6 = 2π/3; d) = - π/3; e) = π/6; ж) 5π/6; з) = 0;
и) 2π/3 +7π/6 = 13π/6; к) 3π/2 - 3π/2 = 0;