Question 1071448
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write the expression as sine, cosine or tangent of a single angle.
cos(pi )/(6)cos(pi )/(7)-sin(pi )/(6)sin(pi )/(7)
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<pre>
Apply the formula of sum for cosine:

cos(a)*cos(b) - sin(a)*sin(b) = cos(a+b).



    See 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.



You will get 

{{{cos(pi/6)*cos(pi/7)) - sin(pi/6)*sin(pi/7)}}} = {{{cos(pi/6 + pi/7)}}} = {{{cos(7pi/42 + 6pi/42)}}} = {{{cos(13pi/42)}}}.
</pre>

Solved.