Trig Identities

**polymerase** · April 21st 2008, 05:50 AM

In a question I was given, in order to solve it, I needed to simplify $\sin\left(\frac{3}{2}x\right)\cos\left(\frac{3}{2} x\right)$ to $\frac{1}{2}\sin 3x$ . This is "obvious" related to the identity $2\sin x \cos x = \sin 2x$ .?

So my question is, what's the rule (if any) to simplify these forms of expressions?

Lets use the example of $\sin\left(\frac{3}{\sqrt{2}}x\right)\cos\left(\fra c{3}{\sqrt{2}}x\right)$ or $\sin\left(\frac{4}{5}x\right)\cos\left(\frac{4}{5} x\right)$ , what do those simplify to?

**Moo** · April 21st 2008, 05:54 AM

Hello,

This is "obvious" related to the identity

Yes

Originally Posted by polymerase

$\sin\left(\frac{3}{\sqrt{2}}x\right)\cos\left(\fra c{3}{\sqrt{2}}x\right)$

It depends on what purpose, sometimes, it's easier not to simplify.

$=\frac{1}{2} \sin \left(2 \cdot \frac{3}{\sqrt{2}} x \right)=\frac{1}{2} \sin \left(3 \sqrt{2} \ x \right)$

Is it what you want ?

or $\sin\left(\frac{4}{5}x\right)\cos\left(\frac{4}{5} x\right)$ , what do those simplify to?

$=\frac 12 \sin \left(\frac{8}{5} x \right) ?$

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