Consider the series 1+(2+3)+(4+5+6)+(7+8+9+10)+(11+12+13+14+15)+....
(a) find an expression for the ith term in the rth bracket.
(b) Given that 39×79 is the sum of all the numbers in the first k bracket in the series, find the value of K.
Well, the 1st term in the rth group is
1,2,4,7,11,...
= 1+ 0,1,3,6,10
= 1+ r(r-1)/2
That should get you started.