# Homework Help: Expressing the limit of a sum as a definite integral

1. Nov 30, 2012

### michaelkorn

1. The problem statement, all variables and given/known data
Express the following as a definite integral:

Express the attached limit as an integral.

3. The attempt at a solution
I have gotten as far as every part of the answer except the upper bound. the answer is:
10
(from 1 to 10) [x-4lnx]dx
1

since the definition of the definite integral is:
a
f(x)dx = lim Ʃ Δxif(x)
b________Δ→∞ i=1

i set Δxi = 9/n since that approaches zero. f(x) would be left to 1+9i/n - 4ln(1+9i/n)
so i set x = 1+9i/n.
since n approaches ∞ and the upper bound of the sum is ∞, i plugged ∞ in for i and n.
thats where I have trouble. ∞/∞ is undefined. when i plug 1 in i end up with 1 so that is the lower bound.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

#### Attached Files:

• ###### Screen Shot 2012-11-30 at 3.05.04 PM.png
File size:
1.9 KB
Views:
150
2. Nov 30, 2012

### Staff: Mentor

I don't think your infinite sum converges for any i - you sum over i which grows like i^2 and the log-expression does not reduce this enough (just grows with i*log(i)).
If the sum is supposed to run from i=1 to n, this makes sense, and you get the maximal x-value simply by setting i=n.

3. Nov 30, 2012

### Dick

There's a typo in the attached limit expression. The upper limit on the summation should be n. As written it doesn't approach anything. The sum by itself diverges.

Last edited: Nov 30, 2012