## EasyMath 2 years ago Rewrite with only sin(x) and cos(x). sin(3x)

1. zepdrix

Oh boy this one is going to be a pain in the butt :) We'll want to apply the Sum Formula for Sine. $\large \sin(a+b)=\sin a \cdot \cos b+\sin b \cdot \cos a$So we can rewrite our problem like this,$\large \sin(3x) \qquad \rightarrow \qquad \sin(x+2x)$See what we're going to do with it?

2. EasyMath

3. zepdrix

No, we're doing the opposite actually.. we STARTED with the X's added together, and we're separating them, writing them as addition. So we can apply the Addition Formula for Sine.

4. EasyMath

Oh okay, so what's next?

5. zepdrix

In this case a=x b=2x And we'll split it up the way I did with the definition above.

6. zepdrix

$\large \sin(x+2x)\quad =\quad \sin x \cdot \cos(2x)+\sin(2x)\cdot \cos x$

7. zepdrix

Try to compare it to the definition above with the sin(a+b), see if it makes sense or not.

8. EasyMath

It doesn't really seem like it makes sense

9. henpen

Of course you will have to repeat the process again to split up sin(2x) and cos(2x), using a=x and b=x.

10. zepdrix

Easy, The thing with the a+b at the very top. That is an identity, you just have to take that for granted. Just believe it!! :D The question is, does our sin(x+2x) match the way we split up the (a+b). Maybe I can give a little more detail.. Hmm

11. zepdrix

$\large \sin(a+b) \quad = \quad \sin(a)\cos(b)+\sin(b)\cos(a)$Try not to get confused by that part ok? You just have to believe that it's true XD In our problem, we've rewritten$\large \sin(3x) \quad \text{as} \quad \sin(x+2x)$We can think of this as $\large \sin(a+b) \quad \text{where} \quad a=x \quad \text{and} \quad b=2x$ $\sin(a)\cos(b)+\sin(b)\cos(a) \quad \rightarrow \quad \sin(x)\cos(2x)+\sin(2x)\cos(x)$

12. zepdrix

See how we replaced the A and B's? Too much for ya? D:

13. EasyMath

No i understand that part completely :p its just it seems like there's something else we have to do because these are my answer choices: 1) 2 sin x cos2x + cos x 2) 2 sin x cos2x + sin3x 3) sin x cos2x - sin3x + cos3x 4) 2 cos2x sin x + sin x - 2 sin3x

14. zepdrix

Yes, we're only half done..... I'm trying to lead you along... I don't wanna go too far into this problem if you're confused.

15. EasyMath

Well I'm with you so far

16. zepdrix

Hmm so we have sin(2x) and cos(2x) terms now. We COULD do the same thing we did before, rewriting it as sin(x+x) and cos(x+x) and applying the sum formulas again. But that would be very tedious. From here, we'd rather apply the Double Angle Formulas. Do you remember those? :D

17. EasyMath

Yes I have them in my notes

18. zepdrix

Ok cool :) So we'll make use of these identities. Sine Double Angle Formula:$\large \sin(2x)=2\sin(x)\cos(x)$ Cosine Double Angle Formula:$\large \cos(2x)=\cos^2(x)-sin^2(x)$

19. EasyMath

Alrighty then

20. zepdrix

$\sin(x)\cos(2x)+\sin(2x)\cos(x)$$\large \rightarrow \quad \sin(x)\left(\cos^2x-\sin^2x\right)+\left(2\sin(x)\cos(x)\right)\cos(x)$

21. EasyMath

Ohh i see

22. EasyMath

So now just multiply them together and you should have your final answer?

23. zepdrix

yes.

24. zepdrix

the sine in the first term needs to be distributed to each term in the brackets. and thennnnn on the right.. i think you end up with a cos^2 since you have 2 cosines and what not.

25. EasyMath

Yes, I understand, thank you very much :)