Write the general solution of tan² 2x = 1.

Write the number of solutions of the equation tan x + sec x = 2 cos x in the interval [0, 2π].

Write the number of solutions of the equation 4 sin x – 3 cos x = 7.

Write the set of values of a for which the equation √3 sin x – cos x = a has no solution.

If cos x = k has exactly one solution in [0, 2 π], then write the value(s) of k.