# Homework Help: Summation in matlab

1. Oct 30, 2009

### sara_87

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

I want to calculate a sum (where the end value is in the sum), eg:
$$\sum^{n-1}_{i=1}{2i+n}$$

2. Relevant equations

I dont want to 'split' the sum, i just want to write this.

3. The attempt at a solution

syms i n
for n=1:5
for i=1:n-1
symsum((2*i+n),i,1,n-1)
end
end

i get the error: ??? Undefined function or method 'symsum' for input arguments of type 'double'.

i think my code is wrong.

2. Oct 30, 2009

### Staff: Mentor

Here's something to try:

Code (Text):

n = sym('n', r);
for n = 1:5
for i = 1:n-1
symsum((2*sym('i', r) + n), i, n-1)
end
end

No guarantee this will work, since I don't have matlab to test it out on. What I did is patterned after the last example on the reference page in the following link. I have also rewritten your syms command to use the sym command. The r flag specifies that both i and n are rational. Without a flag, they default to complex.
Here's a link to a reference page for symsum.
Here's a link to a page for sym.
From the TOC that appears along with the reference page for sym, the next reference page is for syms.

I don't think you have run into it, yet, since you're still trying to get the symsum/syms/sym thing sorted out, but I believe your inside loop will cause problems for you. When n = 1, the inner loop looks like for i=1, 0. The documentation for for doesn't go into as much detail as I would like, so isn't clear what happens when the starting value is larger than the ending value, and you haven't specified a negative increment. Possibly the loop skips that iteration. Don't know.

Anyway, some things to think about and try out.