sn = A0+a 1+a2+……a(n-2)+a(n- 1)

sn = a(n- 1)+a(n-2)+……+a2+a 1+A0

Add the above two formulas:

2sn = (A0+a (n-1))+(a1+a (n-2))+...+(a (n-1)+A0) So: 2Sn = n * (A0+A (n-0/)+A0).

Therefore: Sn=n*(a0+a(n- 1))/2.

Because arithmetic progression's first item+nth item = second item+nth-1item = ... so, the order is reversed and unchanged. Sn can be obtained by adding sequential and reverse permutation.

Understand the formula and do some exercises. It's best not to let "not learning" become an excuse for not doing problems in the future.

Good luck!