# Thread: Help getting a value from a graph.

1. ## Help getting a value from a graph.

Hi.
Like the topic says I need help getting a value from anywhere on a graph I made from a Sigma formula.
The formula goes like this:
0
(Σ 50n)+100
n=9
Giving a start value off 100 and a final value off 2350.
Each point on the graph would be the formula given with n=0-9
Now comes the part where I have no idea what to do, or how to do it. I need to be able to find any value on that graph.
So if I needed to find the value for 5,5 or 7,25 , what would I have to do?

-Pus

2. ## Re: Help getting a value from a graph.

The sum is an arithmetic series. Do you know how to get the closed form of an arithmetic series?

3. ## Re: Help getting a value from a graph.

All I know os that the sum is calculated usimg a arithmetic progression that goes like (n(a1+an)/2

4. ## Re: Help getting a value from a graph.

OK good, so what will be the first term and the last term in your sequence that you are summing up?

5. ## Re: Help getting a value from a graph.

Now I'm not sure if it should be 0 and 9 , 0 and 2250 or 100 and 2350.
But I think it equation would be 100 + (10×((0+9)×50))/2
Where the 100 is just so it starts at 100 when the value is 0.

6. ## Re: Help getting a value from a graph.

Forget about the +100 for the moment. Just deal with the sum. What would be the first term in the sequence and what would be the last term?

7. ## Re: Help getting a value from a graph.

That would be 0 and 9 then I think.

8. ## Re: Help getting a value from a graph.

No, they would be the values of the counter. You are summing up the terms 50n, where the "n" is your counter.

9. ## Re: Help getting a value from a graph.

Ok, then the first and last term has to be 0 and 2250.

10. ## Re: Help getting a value from a graph.

Or is it:
(10×(0×50))/2
and
(10×(9×50))/2
?

11. ## Re: Help getting a value from a graph.

The point Prove it was trying to make is that, since this is an arithmetic series (with 100 added), its sum, up to "n", is $50\frac{n(n+1)}{2}+ 100$. To evaluate that at non-integers, such as 5.5 or 9.5, just use those numbers for n. When n= 5.5 that is $50\frac{5.5(6.5)}{2}+ 100= 50(17.875)+ 100= 17.875$.

12. ## Re: Help getting a value from a graph.

Amazing, it looks like it works just like I wanted it to work. Thank you so much!
And sorry for being bad at math. The last time I had math was the first year at high school. The rest I had to learn by myself.