How do you evaluate Sin(2 arctan(sqrt 2))sin(2arctan(2))?

1 Answer
May 7, 2016

sqrt 2/323.

Explanation:

Let a = arc tan sqrt 2a=arctan2. Then tan a = sqrt 2tana=2. Tangent is positive.

Both sine and cosine have the same sign. So,

either sin a = sqrt2/ sqrt 3 and cos a = 1/sqrt 3sina=23andcosa=13

or sin a = - sqrt 2/ sqrt 3 and cos a = -1/sqrt 3sina=23andcosa=13.

Now, sin (2 arc tan sqrt 2)=sin 2a = 2 sin a cos a = sqrt2 /3sin(2arctan2)=sin2a=2sinacosa=23, in both the cases.