Suite 1 2 10 11 12 13..n

[Z10]

Hello everyone,
I am looking to calculate the sum Sn = 1 + 2 + 10 + 11 + 12 + 13 + 14 .... n
According to n
And calculate n according to Sn after and test the cases where n is not defined.

[Guest]

Sn = 10 + 11 + 12 + 13 + 14 .... n
is an arithmetic sequence.


Its sum is (10 + n)(n - 10)/2

We have to add to that sum 1 + 2 = 3

Or

(10 + n)(n - 10)/2 + 3 = (n^2 - 100)/2 + 3 = n^2/10 - 50 + 3 = n^2/10 - 47

[Guest]

Is it clear?

[Z10]

Yes tnks.