1. Subtract the two items before and after, and 4-1= 3,8-4 = 4,13-8 = 5,3,4,5 constitutes arithmetic progression, so the latter item should be13+6 =15.

2. 1 can be regarded as11,and all molecules are singular, which can be expressed as 2 * n- 1 (n = 1, 2,3, ...); All denominators can be expressed as n 2 (n =1,2,3, ...); So the fifth term n=5, numerator 9, denominator 25, that is, 9/25;