# [SOLVED] Trig Identities (short problem)

• December 5th 2009, 04:18 PM
goliath
[SOLVED] Trig Identities (short problem)
Hey guys, I need to find sin(3 $\pi$ /8)
• December 5th 2009, 04:21 PM
Chris L T521
Quote:

Originally Posted by goliath
Hey guys, I need to find sin(3 $\pi$ /8)

Note that $\sin\!\left(\tfrac{3\pi}{8}\right)=\sin\!\left(\tf rac{1}{2}\left(\tfrac{3\pi}{4}\right)\right)$.

Now apply the half angle formula for sine with $\theta=\frac{3\pi}{4}$.

Can you take it from here?
• December 5th 2009, 04:25 PM
Prove It
Quote:

Originally Posted by goliath
Hey guys, I need to find sin(3 $\pi$ /8)

Alternatively, remember that $\sin{\left(\frac{\pi}{2} - \theta\right)} = \cos{\theta}$.

Can you see that $\frac{\pi}{2} - \frac{\pi}{8} = \frac{3\pi}{8}$?

So that would mean $\sin{\frac{3\pi}{8}} = \cos{\frac{\pi}{8}}$.

And you've been told what that is...