tan A + sec A = 3 { tan^2 A + 1 = sec^2 A → tan^2 A = sec^2 A - 1 → tan A = sqrt [ sec^2 A - 1] }
sqrt [ sec^2A -1 ] + sec A = 3
sqrt [sec^2A - 1 ] = 3 - sec A square both sides
sec^2 A - 1 = 9 - 6sec A + sec^2 A rearrange as
6sec A = 10
sec A = 10 / 6 = 5/ 3
cos A = 1 /sec A = 1 /(5/3) = 3 / 5