cos(A + B): Derives the Cosine of the Sum of two angles which is used to find the other sum and difference identitites
http://www.youtube.com/watch?v=ObaA-rYHO9o
cos(A - B), sin(A + B), sin(A - B): Uses the formula for the cosine of the sum of two angles to derive the formulas for the cosine of the difference of two angles, the sine of the sum of two angles, and the sine of the difference of two angles. Examples using the formula are provided: http://www.youtube.com/watch?v=B9nWZWIN8_I
NOTE: THE ABOVE VIDEO COVERS THE SUM AND DIFFERENCE IDENTITIES FOR SINE, WHICH IS PART OF 3.4.
Cofunction Identities: http://youtu.be/wJ6WTvYA4Yo
Defines cofunctions and develops the six cofunction identities such a sin(π/2 - x) = cos x, etc.
Prove Trig Identity 5: http://youtu.be/PbjYiJ2Pmso
We verify a Trig identity by using the basic identities and the identity for the cosine of a sum or difference. We prove: (cos(x+y))/(cosx·cosy) = 1 - tanx·tany
Prove Trig Identity 6: http://youtu.be/QzB3cTsInpc
We verify a Trig identity by using the basic identities and the identity for the cosine of a sum or difference. We prove: (cos(x+y))/(cos(x — y) = (coty — tanx)/(coty + tanx)