The first is the general formula An= -3n +53 d=An-An- 1=-3.

Then: n "17" means a1"0a2 >, and the first n terms and Sn are both greater than 0; 0.。 . . a 17 & gt； 0;

Then the sum of the first n terms is sn = a 1+a2+...+ An.

=nA 1+(n(n- 1)/2)*d

=50n+(n(n- 1)/2)*(-3)

= The result in the above figure.

You can take a closer look at me and you will understand.