# Integration by parts

1. Jun 8, 2013

### Jbreezy

1. The problem statement, all variables and given/known data

Hi, I attached the question.Just integral trouble.

2. Relevant equations

3. The attempt at a solution

File size:
12 KB
Views:
41
2. Jun 8, 2013

### SteamKing

Staff Emeritus
I don't understand the expression you have written after "This is what I want". You have an integral in the denominator of an expression. I don't see how this is supposed to result in a recursion formula. Have you checked you attachment to make sure all expressions are correctly written?

3. Jun 8, 2013

### Jbreezy

There isn't it is (n-1)/n (times) the integral It isn't supposed to be in the denominator.

∫〖sin〗^n x dx= (-〖sin〗^(n-1) xcosx)/n)+ ((n-1)/n)∫〖sin〗^(n-2) x dx

thx

4. Jun 9, 2013

### haruspex

I find that hard to read. Do you mean $\int \sin^n(x).dx = -\frac1n \sin^{n-1}(x)\cos(x) + \frac{n-1}n \int \sin^{n-2}(x).dx$? Looks right.

5. Jun 9, 2013

### Jbreezy

Yeah that is what I mean.

See I'm trying to figure out how to get to that. On my attachment there is a problem before that where you get

\int \sin^n(x).dx = -\frac1n \sin^{n-1}(x)\cos(x) + \frac{n-1}n \int \sin^{n-2}(x)cos^{2}.dx
They tell you to replace cosx^2 in the second integral and get to
\int \sin^n(x).dx = -\frac1n \sin^{n-1}(x)\cos(x) + \frac{n-1}n \int \sin^{n-2}(x).dx

I'm having trouble with that.

6. Jun 9, 2013

### Jbreezy

How come it doesn't work then I copy and pasted your code for the equation? Anyways my first equation in the attachment I need to get to the one you have written above. Just look at my attachment it is all there/

7. Jun 9, 2013

### haruspex

For some reason, I can never read the .docx files you attach. I see the plain text, but equations and diagrams are blank.
In the latex, you left out the double hash ($) at start and end. I don't understand how you got the first of those two equations. You should have$\int \sin^n(x).dx = [-\sin^{n-1}(x)\cos(x)] + \int (n-1)\sin^{n-2}(x)\cos^2(x).dx$Replacing the cos2 in that gives:$\int \sin^n(x).dx = [-\sin^{n-1}(x)\cos(x)] + (n-1)\int \sin^{n-2}(x)(1-\sin^2(x)).dx\int \sin^n(x).dx = [-\sin^{n-1}(x)\cos(x)] + (n-1)\int \sin^{n-2}(x).dx - (n-1)\int \sin^{n}(x).dxn\int \sin^n(x).dx = [-\sin^{n-1}(x)\cos(x)] + (n-1)\int \sin^{n-2}(x).dx ##
etc.