# Thread: Help to understand one simple function

1. please, can you help me to understand following formula (my math is a bit rusty,....)

I have a following function: where n=2 and −5.0 ≤ xi 5.0

f (x) = n x2i
i=1

So, my question is: since n=2 does it mean that the function needs 2 numbers on input, or just one? I think that only one, is it correct? For example if I use for x number 1 I will get f(x) = 2? And for x=2 I will get f(x) = 8?

thanks,
m.  2.

3. Yes, "n= 2" means that i takes on the values "1" and "2". That "summation form" can be used to write very long sums simply, but with only two terms, it just gets in the way!
That is just requiring two input numbers, and . They are NOT necessarily the same.  4. Originally Posted by HallsofIvy Yes, "n= 2" means that i takes on the values "1" and "2". That "summation form" can be used to write very long sums simply, but with only two terms, it just gets in the way!
That is just requiring two input numbers, and . They are NOT necessarily the same.
many thanks  5. Originally Posted by miro please, can you help me to understand following formula (my math is a bit rusty,....)

I have a following function: where n=2 and −5.0 ≤ xi 5.0

f (x) = n x2i
i=1

So, my question is: since n=2 does it mean that the function needs 2 numbers on input, or just one? I think that only one, is it correct? For example if I use for x number 1 I will get f(x) = 2? And for x=2 I will get f(x) = 8?

thanks,
m.
Your summation is simply a function f(x) = x12 + x22 where x1 and x2 are values in the interval -5 ≤ xi ≤ 5. Are you sure you posted the correct summation? What are you trying to accomplish? You can arbitrarily choose any two numbers in the interval and add their squares.

Jagella  6. The function is a plot of the paraboloid surface for .  7. Originally Posted by Olinguito The function is a plot of the paraboloid surface for .
But there's only one variable, x.

Jagella  8. There are two independent variables, and , and . Thus .

Alternatively, where .  9. Originally Posted by Olinguito There are two independent variables, and , and . Thus .

Alternatively, where .
Thanks, but what happened to the variable, y? You stated earlier that z = x2 + y2.

Jagella  10. Sorry, clash of notation. The OP used to represent a point in whereas I used for such a point. The two represent different variables.

NB: The OP only wrote and did not actually say the variable was. I’m assuming because it’s seems to me the most logical assumption to make.  Bookmarks
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