MATLAB Answers

## Compute the infinite sum of pi/4 up to 5 correct decimals.

Asked by Snirisa Gödel

### Snirisa Gödel (view profile)

on 19 Sep 2013
Accepted Answer by Azzi Abdelmalek

### Azzi Abdelmalek (view profile)

I want to approximate pi/4 by using the infinite sum pi/4=((-1)^(n))/(2*n+1) from 0 to infinity. Here is my MATLAB code:

```if true
tol = 10^(-5)      %Up to five correct decimals
d=1;               %Arbitrarily chosen, only condition is that d>tol at the start
sn=0;              %Initial starting value
n=0;
while d>tol
```
`    sn = ((-1)^(n))/(2*n+1);  This is the approximation for pi/4.`
```    d=abs(pi/4-sn);           Value that decides whether the loop continues.
n=sn;                     I'm not sure but I want n to increase by one each     ```
```                              time to prevent
circular calculations.```
`    end`

Why do I get Inf+ Nani?

dpb

### dpb (view profile)

on 19 Sep 2013

Why do I get Inf+ Nani?

Because of

`   n=sn; `

Try each step directly at the command line and see what happens...

You need to set a value for the total of the summation of each term initially to zero and then accumulate the terms into that variable -- sn is ok for the name but you don't accumulate a sum of the terms in n, you replaced in with the individual term going forward.

To get the subsequent terms you need to increment n, that is correct --

n=n+1;

where the n=sn; line is instead; the implementation of accumulating the summation is left for you to consider how to do that a little more...

Snirisa Gödel

### Snirisa Gödel (view profile)

on 19 Sep 2013

Thank you dpb, I appreciate you're answer.

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## 1 Answer

### Azzi Abdelmalek (view profile)

Answer by Azzi Abdelmalek

### Azzi Abdelmalek (view profile)

on 19 Sep 2013
Edited by Azzi Abdelmalek

### Azzi Abdelmalek (view profile)

on 19 Sep 2013
Accepted answer

```tol = 10^(-5)
d=1;
sn=0;              %
n=0;
while d>tol
sn = sn+(-1)^n/(2*n+1);
n=n+1;
d=abs(pi/4-sn);
end
disp(sn)
```

Jan Simon

### Jan Simon (view profile)

on 19 Sep 2013

This is obviously a homework question. Solving it does not allow the OP to find the solution by his own.

Azzi, please do not post solutions of homework questions, because this reduces the reputation and efficiency of the forum. As a teacher on a university you should think of your colleagues, who do not want to get solutions created by others than their students.

Snirisa Gödel

### Snirisa Gödel (view profile)

on 19 Sep 2013

Thank you.

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