1. ## Evaluate trigo expression

How to evaluate: Sinx - 2sin4x + sin6x

Hello owen8810

Welcome to Math Help Forum!
Originally Posted by owen8810
How to evaluate: Sinx - 2sin4x + sin6x
Sorry, I don't understand what is wanted here. Could you give us the original question, please?

Hello owen8810

Welcome to Math Help Forum!Sorry, I don't understand what is wanted here. Could you give us the original question, please?

evaluate or simplify: sin4 - 2sin4x + sin6x

thks.

4. ## Trig Formulae

Hello owen8810
Originally Posted by owen8810
evaluate or simplify: sin4(?) - 2sin4x + sin6x

thks.
This doesn't really help, especially as you have a typo in your reply.

Evaluate and simplify mean two different things: evaluate means 'find the value of', and simplify means 'write in a simpler form'.

We can only evaluate a function of x if we know a particular value or values of x to substitute into the function. So that still doesn't make sense here.

The only way you could simplify an expression like this is to use trig formulae to write it in an alternative form, and hope that you can then write the result more simply. I can think of two possibilities here:

• Use formulae for $\sin 4x$ and $\sin 6x$ in terms of single angles; you'll find the formulae here, for example.
• Re-write $\sin x - 2\sin4x+\sin 6x$ as $(\sin x - \sin 4x) + (\sin 6x - \sin 4x)$, and then use the formula $\sin A - \sin B = 2\cos\tfrac12(A+B)\sin\tfrac12(A-B)$ for each part of the expression.

I have tried both of these approaches, and I can't see that the results are any simpler than the original expression! So unless we have some more information, I can't see how we can proceed further.