Question

if 3 is the first triangular no which is the 8th one

Answer 1

Answer:

45

Step-by-step explanation:

Triangular numbers are 1,3,6,10, 15,21,28,36,45,55,66, 78,91……… n th Number

derived from the formula n(n+1)/2

But in the given problem 3, which is a Second Triangular number , is considered as First triangle number.

hence 9th number will be 8th number i.e,

9(9+1)/2 = 9×10/2 = 45