From the sequence given:
-1,0,3,8,15...
the difference between the terms is:
1,3,5,7,..
the above numbers are add numbers, thus the series if obtain by adding add number to the terms. Thus the formula will be:
an=a(n-1)+i
where n is the nth term
a1=-1
i takes all the off integers, i=1,3,5,...
