Question 773663
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{{{7*pi/12 = pi/3 + pi/4}}}
{{{cos (7*pi/12) = cos (pi/3 + pi/4) = cos(pi/3)*cos(pi/4) - sin(pi/3)*sin(pi/4)}}}
pi/4 = 45 deg. {{{sin (pi/4) = cos(pi/4) = 1/sqrt(2)}}}
pi/3 = 60 deg. {{{sin (pi/3) = sqrt(3)/2}}} and {{{cos(pi/3) = 1/2}}}

So {{{cos (7*pi/12) = (1/2)*(1/sqrt(2)) - (sqrt(3)/2)*(1/sqrt(2))}}} =
{{{(1 - sqrt(3))/(2*sqrt(2))}}}
Hope you got it :)
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