# Riemann Sum-Area

1. Apr 29, 2006

### NIZBIT

I was hoping someone could check my answer?

Use the limit of a Riemann sum to find the area of the region bounded by the graphs of y=2x^3+1, y=0, x=0, x=2.

Area=2

2. Apr 29, 2006

### Jameson

The limit of the Riemann Sum is the integral. So evaluate $$\int_{0}^{2}2x^3+1dx$$. I don't get 2. How did you do your problem?

3. Apr 30, 2006

### benorin

A useful formaula is

$$\int_{a}^{b}f(x) \, dx = \lim_{n\rightarrow\infty} \frac{b-a}{n}\sum_{k=1}^{n}f\left( a+\frac{b-a}{n}k\right)$$​

hence

$$\int_{0}^{2}(2x^3+1) \, dx = \lim_{n\rightarrow\infty} \frac{2}{n}\sum_{k=1}^{n}\left[ 2\left( \frac{2k}{n}\right) ^3 +1\right]$$​

4. May 1, 2006

### NIZBIT

Now I am getting two answers-10 for the def integral and 8 for the sum.

5. May 1, 2006

### HallsofIvy

Staff Emeritus
10 is obviously the correct answer. How are you taking the limit, as n goes to infinity on
$$\lim_{n\rightarrow\infty} \frac{2}{n}\sum_{k=1}^{n}\left[ 2\left( \frac{2k}{n}\right) ^3 +1\right]$$
?

Do you know a formula for the sum of k3, k2, and k that you are using?