Помогите решить

1) 7tg - 2ctg + 5=0

2) sin5x+sin7x=0

3) 3cos2x-19cosx+6=0

4) cosx-21sinx-9=0

1) 7 tg x - 2 ctg x+5=0

7/ctg x - 2 ctg x+5=0

(7-2ctg^2 x + 5 ctg x) / ctg x=0

7-2ctg^2 x + 5 ctg x=0

-2ctg^2 x + 5 ctg x+7=0

2ctg^2 x-5 ctg x-7=0

ctg x = a

2a^2-5a-7=0

D = (-5) ^2 - 4*2 * (-7) = 25+56=81 V (корень) D=9

x1 = (5+9) / 4=3,5

x2 = (5-9) / 4=-1

ctg x = 3.5 - решений нет

ctg x = - 1

x = - pi / 4+pi*n, neZ

2) sin5x+sin7x=0

2sin6x*cos2x = 0

2sin6x = 0 или cos 2x = 0

a) 2sin6x = 0

sin 6x=0

x = (pi*n) / 6, neZ

b) cos 2x = 0

x=pi/4 + (pi*n) / 2, neZ