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Question #ea04c

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Mar 21, 2017

$cos (pi/7 )cos ((4pi)/21) - sin (pi/7) sin ((4pi)/21)$

$=cos (pi/7 +(4pi)/21)$

$=cos( (3pi +4pi)/21)$

$=cos( (7pi)/21)$

$=cos( (pi)/3) =1/2$

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