Вот теперь другое дело.
13^(sin^2 x) + 13^(cos^2 x) = 14
13^(sin^2 x) + 13^(1 - sin^2 x) = 14
13^(sin^2 x) + 13/13^(sin^2 x) = 14
Замена 13^(sin^2 x) = y
y + 13/y = 14
y^2 - 14y + 13 = 0
(y - 1)(y - 13) = 0
1) y = 13^(sin^2 x) = 1
sin^2 x = 0
x1 = pi*k
2) y = 13^(sin^2 x) = 13
sin^2 x = 1
1 - cos^2 x = 1
cos^2 x = 0
x2 = pi/2 + pi*k