How do you express cos( (5 pi)/4 ) * cos (( 11 pi) /6 ) without using products of trigonometric functions?

1 Answer
Nghi N.
May 17, 2016

-sqrt6/4

Explanation:

Product P = cos ((5pi)/4).cos ((11pi)/4)
Trig table and unit circle -->
cos ((5pi)/4) = cos (pi/4 + pi) = - cos (pi/4) = - sqrt2/2
cos ((11pi)/6) = cos (-pi/6 + (12pi)/6) = cos (-pi/6 + 2pi) =
cos (-pi/6) = cos (pi/6) = sqrt3/2
Therefor:
P = (-sqrt2/2)(sqrt3/2) = - sqrt6/4

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