What is the rule for this function?

Question

Input 1 output 3
Input 2 output 5
Input 4 output 9
Input 5 output 11
Input 10 output 21
Input 12 output 25
Please help thanks

Answer 1

http://www.jiskha.com/display.cgi?id=1354418711

Answer 2

an = 2 * n + 1

For n = 1

a1 = 2 * 1 + 1 = 2 + 1 = 3

For n = 2

a2 = 2 * 2 + 1 = 4 + 1 = 5

etc.