Proving this trig identity

1. May 2, 2005

relativitydude

This is a tricky one, sin(3x)cos(x)=sin(x)cos(x)(3-sin^2(x))

I have tried addition formulas and expanding sin(3x) into something else, any help?

2. May 2, 2005

HallsofIvy

Staff Emeritus
That's not just "tricky", it's false! Try $$x= \frac{\pi}{4}$$.

3. May 2, 2005

Pyrrhus

Like Halls said, remember that an identity is an equation that all values are solutions.

4. May 3, 2005

dextercioby

It should have been

$$\cos x\sin 3x\equiv \cos x\sin x\left(3-4\sin^{2}x\right)$$

Daniel.