the natural numbers are grouped as follows:

(1) , ( 2 , 3 ) , ( 4 , 5 , 6 ) , ( 7 , 8 , 9 , 10 ) , ...

find an expression for the first term of thenth group and prove that the sum of the numbers in thenth group is 1/2 n (n^2+1).

uh...I have no idea where to even start.

thank you in advance!