How do you express $cos( (5 pi)/6 ) * cos (( 7 pi) /4 )$ without using products of trigonometric functions?

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Answer 1 · 2016-01-26T06:11:36

$( -sqrt6/4)$

Trig table of special arcs -->
$cos ((5pi)/6) = - sqrt3/2$
$cos ((7pi)/4) = cos (- pi/4 + 2pi) = cos (-pi/4) = cos (pi/4) = sqrt2/2$

$cos ((5pi)/6).cos ((7pi)/4) = - (sqrt3/2)(sqrt2/2) = -sqrt6/4$

How do you express cos( (5 pi)/6 ) * cos (( 7 pi) /4 ) cos( (5 pi)/6 ) * cos (( 7 pi) /4 ) without using products of trigonometric functions?