Posted by lindsay on Friday, May 4, 2012 at 4:30am.
I need help in understanding how to identify which identity to use. using this question as an example.
Show that tan theta (sin theta+cot theta cos theata)all over cos theta = sec^2 theta. Please explain.

trigonometry  Steve, Friday, May 4, 2012 at 4:41am
tan(sin+cot*cos)/cos
When dealing with such a combination of functions, it is usually good to start out by converting everything to sin and cos:
(sin/cos)(sin+(cos/sin)cos)/cos
sin/cos * (sin^2 + cos^2)/sin * 1/cos
(sin^2 + cos^2)/cos^2
1/cos^2
sec^2