(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

Find the sum formulas for cosine and sine using vector methods.

2. Relevant equations

Suggestion: use the following vectors

A= cos xi+ sin xj

B= cos yi+ sin yj

C= cos yi- sin yj

3. The attempt at a solution

I actually solved the question by doing the dot product of A and B and finding the cosAB = cos (x-y). Then, I changed y to -y and obtained the other formula concerning the sum of cosines. The same thing was done with the cross product of A and B in order to obtain the formulas for sum and subtraction of sines. I am not completely satisfied with this solution, since the author gives the suggestion to use a vectorC. I tried doing the dot product of A and C in order to obtain cos (x+y), but it did not work out since the length of vector C is cos(2y). Same thing happened when I did their cross product to obtain sin (x+y). I am thinking I am making some kind of geometrical mistake, but I have not found what would that be. Can anyone help me on this?

Thanks

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# Homework Help: Vector analysis 2

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