tan²a + sec a = 5, tentukan nilai cos a

tan²a = sec²a - 1 (identitas trigonometri)
sec²a - 1 + sec²a - 5 = 0
sec²a + sec a - 6 = 0
(sec a - 2)(sec + 3) = 0
sec a = 2 atau sec a = -3
cos a = 1/sec a
cos a = 1/2
atau
cos a = 1/-3 = -1/3