# Integrate 3cos^2(x)

1. Apr 14, 2009

### ZaZu

~~ Integrate 3cos^2(x)

Hey guys,

Can you please show me a step by step integration for

Find the solution for the differential equation :

3Cos2(x) , y= Pi , x = Pi/2

Thank you !

2. Apr 14, 2009

### danago

Re: ~~ Integrate 3cos^2(x)

To integrate (cos x)^2, can you think of a trig. identity than can be used to change it to a form that should be easy to integrate?

3. Apr 14, 2009

### ZaZu

Re: ~~ Integrate 3cos^2(x)

Ummm Cos2(x) ?? =\

Sin^2(x) ?

To be honest I do not know, thats why I wanted help.

How would I know what Trig function would I need to integrate that ?!

Last edited: Apr 14, 2009
4. Apr 14, 2009

### ZaZu

Re: ~~ Integrate 3cos^2(x)

Someone please help me, I have an exam tomorrow and there are several questions with the same style !

5. Apr 14, 2009

### danago

Re: ~~ Integrate 3cos^2(x)

Ok, perhaps start from cos(2x).

Do you know the "double angle formula"? How can you write cos(2x) in terms of just cos(x)? Try to figure that out, and it should be clear what to do next after that.

6. Apr 14, 2009

### User Name

Re: ~~ Integrate 3cos^2(x)

danago, he's asking for cosine squared of x, not cosine of 2x.

ZaZu, have you learned integration by parts?
That's really the only way I can see you integrating this function.

Set u = 3cos^2(x) and dv = dx.

Solve from there.

7. Apr 14, 2009

### Hootenanny

Staff Emeritus
Re: ~~ Integrate 3cos^2(x)

User Name, it is far more straight forward to transform cos2x into cos(2x) and integrate from there rather than integrating cos2x using integration by parts, which is messy.

8. Apr 14, 2009

### ZaZu

Re: ~~ Integrate 3cos^2(x)

Integration by parts will prompt a more complicated answer :S I tried it and its too messy

I want to know how do we convert Cos^2x into Cos(2x) .. whats the relationship between that ?? Cos2x is a double angle, and cos^2(x) is Cosine Squared ... Arent they both different ?

Please clarify that to me !

9. Apr 14, 2009

### danago

Re: ~~ Integrate 3cos^2(x)

Cos(2x) = Cos2(x)-Sin2(x) = 2Cos2(x)-1

Can you see how to use that?

10. Apr 14, 2009

### User Name

Re: ~~ Integrate 3cos^2(x)

Ah, my mind completely skipped over that trigonometric property, and simply thought that danago had misread ZaZu's original problem.

My apologies.

Anyway, danago is pushing you towards the correct answer, ZaZu.

11. Apr 14, 2009

### ZaZu

Last edited by a moderator: May 4, 2017
12. Apr 14, 2009

### danago

Re: ~~ Integrate 3cos^2(x)

How did you get from your first to second line?

Cos2(x)=Cos(x)Cos2(x)?

From the trig identity i posted, you can change to:

Cos2(x)= [Cos(2x)+1]/2

Which is much easier to integrate.

Last edited by a moderator: May 4, 2017
13. Apr 14, 2009

### ZaZu

Re: ~~ Integrate 3cos^2(x)

Yeah I mentioned it up there, I just realized I solved for Cos^3(x) xD

The thing is, I dont understand WHY would I convert it into cos2x even if I used the trig function of Cos2(x) = 2Cos^2(x) - 1 ..

14. Apr 14, 2009

### danago

Re: ~~ Integrate 3cos^2(x)

Simply because it is much easier to integrate.

3Cos2(x)= 3[Cos(2x)+1]/2 = (3/2) [Cos(2x) + 1]

Now you can integrate easily using the fact that $$\int Cos(ax) dx = (1/a) Sin(ax)+C$$

15. Apr 14, 2009

### ZaZu

Re: ~~ Integrate 3cos^2(x)

So you're saying that its a rule I should memorize ??

16. Apr 14, 2009

### danago

Re: ~~ Integrate 3cos^2(x)

The trig identity? You should learn (either memorize or learn to derive) the common trig identities because they can often be used to make an integration much easier to perform by allowing you to change the integrand to something "nicer". Trig identities are very useful to know, not just for intergration, but for math in general.

17. Apr 14, 2009

### ZaZu

Re: ~~ Integrate 3cos^2(x)

I do know some of the trig identities, but I do not understand how using them here can help me.

But I came up with this rule : If the index is an EVEN number, I double the angle.
If the index is an ODD number, I use the trig identities to substitute instead.

Cos^2(x) = Cos(2x)

Cos^3(x) = Cos(x) x Cos^2(x)
..............= Cos(x) x (1 - Sin^2(x) ) ... etc

18. Apr 14, 2009

### Cyosis

Re: ~~ Integrate 3cos^2(x)

I don't want to sound like an ***, but reading carefully is a very important part of mathematics and science. Multiple people have given you the answer yet you seem to just ignore it and come up with a rule of yourself that is plainly wrong. Coming up with rules yourself is very good, however do try to prove them so you know they are correct.

Could you solve this equation for $\cos^2(x)$?

19. Apr 14, 2009

### ZaZu

Re: ~~ Integrate 3cos^2(x)

Cyosis im having a difficulty understanding what they are telling me, I do not know why, I thinks its nervousness prior to exams. Im panicking ..

Are you asking me to solve Cos^2(x) as in integrate Cos^2(x) ?

By the way, Im sorry everyone, my brain is just not tolerating a thing anymore .. :(

20. Apr 14, 2009

### danago

Re: ~~ Integrate 3cos^2(x)

That first line is not correct. Cos2(x) is not the same as cos(2x).

with the trig identity, what i am saying is that cos2(x) is equivalent to [cos(2x)+1]/2 for all values of x -- they are practically the exact same things just written differently (thats what an identity is). Hence integrating the second form will effectively give you the exact same result as integrating the first form, only difference is that the original form is a lot harder to do.