Question 898128
Layer Number__________Balls in Layer
1_______________________1
2_______________________3
3_______________________6
4______________________10
5______________________15__________added 5
6______________________21__________added 6
.
.
n__________________sum of 1 to n, the number of balls in a layer.


{{{sum(i,1,n)}}}, the number of balls in the layer at index n.


The formula for this kind of summation is  {{{(n/2)(First+Last)}}} for the SEQUENCE on which the summation is based.


{{{highlight((n/2)(1+n)=8436)}}}.
Solve for n, which is also the number of horizontal layers.