Make a list of all definitions in the section (a few words each is fine). Summarize any lengthy definitions introduced in this section in your own words.
You should know the definitions of the various ``secondary'' trig functions,
which are defined in terms of 
and 
:
Make a list of all theorems (lemmas, corollaries) in the section (a few words each is fine). Summarize each one introduced in this section in your own words.
This makes sense graphically, since if we zoom in on the function 
as
, it appears more and more like the line y=x. Thus the ratio should
be approaching 1, since they're getting more and more similar as .
Make a note of any especially useful properties, tricks, hints, or other materials.
Identity:
This identity comes from the Appendix D, which you may want to consult if you're a little shaky on your trig functions.
The author notes that the derivatives of the ``co'' functions are the ones with the negative signs.
Summarize the section in two or three sentences.
In this case we use a few tricks and identities to find the derivative functions for the trigonometric functions. Since all these functions are defined in terms of sines and cosines, if we know the derivatives for these the others can be derived using the quotient rule.