Use the half angle formula to find the exact value of cos(-3pi/8)

Question

anonymous · Answer

$$\cos 2\theta = \cos^2 \theta - \sin^2 \theta = 1- 2\sin^2\theta = 2\cos^2 \theta - 1$$

Try to use any one of the above formula and get the answer...

anonymous · Answer

Firstly you will use this formula :

$$\cos (- \theta) = \cos \theta $$

So,
$$\cos(-3 \pi /8) = \cos (3 \pi /8)$$

Now use the above identity...

anonymous · Answer

thats actually for the double angle formula isnt it? Half angle is +-sq.rt. 1-cos(a)/2?

anonymous · Answer

Yes buddy you are given cos2x

Just find the cosx or sinx from the identities I have given...

anonymous · Answer

ok after plugging root 2/2 into the formula I got $$(\sqrt{2-\sqrt{2}})/4$$

anonymous · Answer

Buddy, thanks I appreciate you for doing this..
You are absolutely correct...