Question 1007358
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Do you know this formula 

{{{cos(alpha)*cos(beta) - sin(alpha)*sin(beta)}}} = {{{cos(alpha+beta)}}} ?


It is standard formula of Trigonometry. It can be found in any textbook in Trigonometry. 

See, for example the lesson <A HREF=http://www.algebra.com/algebra/homework/Trigonometry-basics/Addition-and-subtraction-formulas.lesson>Addition and subtraction formulas</A> in this site.

Use it, and your life will be much easier.
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