I'm struggling in marked question in this exercise. I already have the answer provided by the book but I did not know the process to get that number. Please help me, thank you much!

13. Converges; compare to 1 72, as 1/ (n2 + 3n - 5) 1. 15. Diverges; compare to 1 =, as 1 3. n n=1 17. Diverges; compare to 1 IM -. Since n = Vn2 > Vn2 - 1, n 1 2. 19. Diverges; compare to n=1 n2 tn+ 1 notn+1 1. 21. Diverges; compare to - . Note that n n=1 n 1 n2 - 1 12 1 n as > 1, for all n > 2. OO\fOO 14. 1 40 + 12 - n 0=1 15 Inn n n=1 1 16. IM 8 n! + n 1 17 M 8 1= 2 Vn2 - 1 18. n - 2 n2 tn+ 1 19. n3 - 5 n=1 OO 20 20 57 + 10 n=1 OO n 21. n2 - 1 n=2