Question

Find x in the series 1,3,6,10,15,..............,x
Where x is the 15th term in the series

Answer 1

the sequence of triangular numbers 1, 3, 6, 10, 15, 21, 28... is given by 12n(n+1)  12⋅11⋅(11+1)+12⋅12⋅(12+1)
=12⋅11⋅12+12⋅12⋅13
=66+78=144=122