Step-by-step explanation:
A1=8
A2=A1+5 plug 8 into A1 here. After getting the value, put into next equation. And repeat until you get A5
A3=A2+5
A4=A3+5
A5=A4+5
I believe that it would be 13, unless you were trying to make the 9 negative. Hope that helps.
Answer:
Proof for 
Step-by-step explanation:
We are given that a, n, b, r and k are integers.
Also,

Since k divides a and b, we can write,
a = rk and b = sk, where r and s are integers.
Now, we have to prove that k divides r as well that is 
Putting value of a and b in the equation, we get:

Since, (r-sn) is an integer, k divides r.