1, according to the quadratic function image, sn is the smallest when n is equal to 8.5, but in the series, n must be an integer, and according to symmetry, sn is the smallest when n is equal to 8 or 9.

2, the sequence is: 7, 14, 2 1. . . . 9 1, 98, a *** 14, and the sum of them is 735 according to arithmetic progression.

3，s5=a 1+a2+a3+a4+a5=5*a3。 So a3 is equal to 8, and because a2+a5= 19=a3+a4, a4 is equal to 1 1. According to a3, it is equal to 8 and a4 is equal to 65438.