Proof Sn Arithmetic Series

Proof sum to n terms for an Arithmetic series

[HaydenJory]

-Arithmetic sequence is a linear function
Tn=a+(n-1)d
Tn=a+dn-d

-Sum of an arithmetic sequence is a quadratic function
C equals zero
An²+bn
na+(.5n²-.5n)d

The functions intercept at T1=S1
na+(.5n²-.5n)d

[abhipenikelapati]

Why do you add the original order with the reversed order? How does it work?

[katialourenco]

Thank you so much for your help❤ it is very much appreciated 🫂