делим на cos^2x

tg^2x - 2tgx - 3 = 0

tgx = a

a^2 -2a -3 = 0

2±√4+12 /2

a1 = 2+4/2 = 3

a2 = 2-4/2 = -1

1) tgx = 3

x = arctg3 + πn

2) tgx = -1

x = -π/4 +πn