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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 ?

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    see example at: http://en.wikipedia.org/wiki/Riemann_sum2012-09-05
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    It's not clear to me whether you don't know the formula for the left rectangle sum, or whether you know it but you don't know how to get to it. To derive a formula means to show how someone might arrive at that formula. Roughly speaking, it means to prove that the formula is correct.2012-09-05
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    Yea I can work these problems out, it's pretty straightforward compared to the rest of calculus . But I'm not good at proofs though like don't even understand what its asking me to put down at all.2012-09-05

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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