Thread: Trig identities.

Hello everyone, I have a couple of questions for you today.

First, how does one deal with "sinx + sin2x" (or anything for that matter, cosx - cos2x, etc)? Is it "sin3x" or rather sinx + sin(x+x) and then expand the sin(x+x) into sinxcosx+cosxsinx?

An Identity to fit the question,

(sinx-sin3x)/(cosx+cos3x) = -tanx
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Also, this question on my review homework seems to be ridiculous, and no one in my class can seem to get it.

(sinx+siny)/(cosx-cosy) = -cot(x/2 - y/2)

May I also mention, we can not use the Sum-product identity, cosa + cosb = 2cos(-------,

Thank you

You have the right idea. Changing an odd "2x" function to a bunch of 'x' functions should be beneficial.

For that last one, there MUST be a clue in the expansion of tan(A+B).