[SOLVED] Trig Identities (short problem)

**goliath** · December 5th 2009, 04:18 PM

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

**Chris L T521** · December 5th 2009, 04:21 PM

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?

**Prove It** · December 5th 2009, 04:25 PM

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...

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