Question

What is the closed linear form for this sequence given a1 = 13 and an + 1 = an + 4?

Answer 1

I am assuming that  the last part is 

a(n+1) = a(n) + 4     so each term successive increases by  4

y = 4x + b 

when x = 1, y = 13 so  b = 9
y = 4x + 9

or 

a(n) =  4n + 9