Question 1060407
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{{{cos(2pi/5)*cos(pi/15) + sin(2pi/5)*sin(pi/15)}}} = {{{cos(2pi/5-pi/15)}}} = {{{cos(6pi/15-pi/15)}}} = {{{cos(5pi/15)}}} = {{{cos(pi/3)}}} = {{{1/2}}}.


The sum or difference formulas for cosine are

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

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