httpsgyazocom4da0fe96d58971536e9ddebe2fdb2dc8Solutioncos pi

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cos ( pi/8 ) cos (6pi/7 )

cos^2 pi/8 = ( 1+ cos (pi/4 ) / 2

by half angle formula

cos pi/8 = sqrt ( 2 + sqrt 2 ) / 2

cos 6pi/7 cannot be simplified further using sum or difference formula or half angle formula ,

so this can be written as

sqrt ( 2 + sqrt 2 ) / 2 (cos (6pi/7) )

0.90096886 * sqrt ( 2 + sqrt 2 ) / 2