definite integral....finding the function

The following sum

$\displaystyle \frac{1}{1+\frac{3}{n}} \cdot \frac{3}{n} + \frac{1}{1+\frac{6}{n}} \cdot \frac{3}{n} + \frac{1}{1+\frac{9}{n}} \cdot \frac{3}{n} + \ldots + \frac{1}{1+\frac{3 n}{n}} \cdot \frac{3}{n} $

is a right Riemann sum for a certain definite integral $\displaystyle \int_1^b f(x)\, dx $

using a partition of the interval [1,b] into n subintervals of equal length.

i found b to be 4 but i cannot figure out what f(x) is.

i thought it was 1/(1+x) but thats not it =(

help please. thank you