derive the formula for the left rectangle sum $f(x)=x^2+1$ from $0$ to $3$ Simply that, derive the formula for the left rectangle sum $f(x)=x^2+1$ from $0$ to $3$
This is when you use like rectangles and Riemann sums to approximate an integral.  Not really sure what this means to derive the formula ?

Can you follow the very similar example on this web site?
http://www2.seminolestate.edu/lvosbury/CalculusI_Folder/RiemannSumDemo.htm
It is important for you to learn what is going on here. 
I would strongly recommend you use all three (left, right and midpoint) to find the integral.
Of course, you know what the answer should be by doing the integral.
Hint 1: Area = 12
Hint 2: See the left sum here: 
http://www.wolframalpha.com/input/?i=INTEGRATE%5Bx%5E2%2B1%2C%7Bx%2C0%2C3%7D%5D&t=crmtb01
Please show your work if this is confusing.
HTH ~A
