Тригонометрия, никак не могу вникнуть, с чего начать с объяснением.

cos(a) ≠ 0

1 + 1/cos(a) + tg(a) = cos(a)/cos(a) + 1/cos(a) + sin(a)/cos(a) = (cos(a) + sin(a) + 1)/cos(a)

1 - 1/cos(a) + tg(a) = cos(a)/cos(a) - 1/cos(a) + sin(a)/cos(a) = (cos(a) + sin(a) - 1)/cos(a)

(cos(a) + sin(a) + 1) (cos(a) + sin(a) - 1) =  (cos(a) + sin(a))² - 1 = cos²(a) + 2sin(a)cos(a) + sin²(a) - 1 = (cos²(a) + sin²(a) = 1) = 2sin(a)cos(a)

cos(a)*(1 + 1/cos(a) + tg(a))*(1 - 1/cos(a) + tg(a)) = cos(a) *2sin(a)cos(a)/ (cos(a)*cos(a)) = 2sin(a)

ответ А