What is the next number in this sequence 1,2,6,15,31

The general form of the quetion of a circle is ax^2+by^2+cx+dy+e=0 where a=b=0 if the circle has a radius of three units and the center lies on the y axis which set of values of a, b, c, d, and e might correspond to the circle

Any is very appreciated! (picture is provided below)not sure how to do this at all won't lie

Abcd is a square. what is the measure of angle bac