Triangular numbers can be represented with equilateral triangles formed by dots. the first five triangular numbers are 1, 3, 6, 10, and 15. is there a direct variation between a triangular number and its position in the sequence? explain your reasoning.

Position in the sequence    triangular number    relation

1                                          1                              1 = 1*(1+1)/2

2                                          3                              3 = 2(2+1)/2

3                                          6                              6 = 3(3+1)/2

4                                         10                            10 = 4(4+1)/2

5                                         15                              15= 5(5+1)/2

Call n the position in the sequence, then the triangular number is: n(n+1)/2
 

The answer is $9.90 with rounding to the nearest tenth