#f(x) = sin x - sin x/cos x = sin x*(1 - 1/cos x) = sin x*(cos x - 1)/cos x.#
Solve #sin x = 0# and #(cos x - 1) = 0#
Reminder. #f(x)# undefined when #cos x = 0 --> x = pi/2 and (3pi)/2.# #sin x = 0 --> x = 0; x = pi#; and #x = 2pi# #cos x - 1 = 0 --> cos x = 1 --> x = 0; and x = 2pi#
Answers within period #(0, 2pi): 0, pi, 2pi#.
Check: #x = pi --> f(x) = 0 - 0 = 0# (correct)