This question is about Pentagonal Numbers, written Pn where P1=1, P2=5, P3=12, P4=22, P5=35, P6=51...........

I am confused about the three questions listed below. Thank you so much for your time.

(a) Find a closed form for the pentagonal numbers. (b) Let r and y be integers. Find variables X and Y in r and y respectively, as well as an integer D such that a solution to Pr = y gives rise to a solution to X2 - DY? = 1.1 (c) Find a way to classify all such solutions to X2 - DY? = 1, and use that to find all pentagonal numbers that are also squares