Mark M. answered 11/09/18
Tutor
4.9
(954)
Retired math prof. Very extensive Precalculus tutoring experience.
(tanu)(sinu) + cosu = (sinu)/(cosu)(sinu) + cosu
= sin2u / cosu + cosu
= [sin2u + cos2u] / cosu
= 1 / cosu = secu