1) 60° это π/3; 144° - это 144·π/180 = 4π/5; 3π/4 - это 135°; 5π/18 - это 50° 2) Сos a = 2/√5 Sin²a + Cos² a = 1 Sin²a + 4/5 = 1 Sin² a = 1/5 Sin a = 1/√5 tg a = Sin a/Cos a = 1/√5 : 2/√5 = 1/2 3) Sin²t/(1 + Cos t) + Sin t·Cos t/Sin t =Sin²t/(1 + Cos t) + Cos t = = (Sin²t + Cos t + Cos²t)/(1 + Cos t) = (1 + Cos t)/(1 + Cos t) = 1 4) Ctg t = -21/20 ⇒ tg t = -20/21 1 + tg² t = 1/Cos² t 1 + 400/441 = 1/Cos²t 841/441 = 1/Cos²t Cos²t = 441/841 Сos t = 21/29 Sin² t + Cos²t = 1 441/841 + Cos²t = 1 Cos²t = 1 - 441/841= 400/841 Cos t = - 20/29