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I am trying to study for a test and the teacher suggest we memorize $\sin(A+B)$, $\sin(A-B)$, $\cos(A+B)$, $\cos (A-B)$, and then be able to derive the rest out of those. I have no idea how to get any of the other ones out of these, it seems almost impossible. I know the $\sin^2\theta + \cos^2\theta = 1$ stuff pretty well though. For example just knowing the above how do I express $\cot(2a)$ in terms of $\cot a$? That is one of my problems and I seem to get stuck half way through.

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    What textbook do you use? I'm sure that in your textbook all the formulas are proved using only the four formulas mentioned above.2011-07-01
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    I don't have a text book, it got lost in the mail. I am more concerned with learning how to actually do it though, it seems impossibly hard to me.2011-07-01
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    Try http://www.sosmath.com/trig/trig.html2011-07-02

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