Question

Write a rule for each pattern. Then use your rule to find the next two terms in the pattern. The numbers are: 1, 3, 6, 10, 15, 21, 28, 36

Answer 1

Triangular Number Sequence

This is the Triangular Number Sequence:

1, 3, 6, 10, 15, 21, 28, 36, 45, ...

This sequence comes from a pattern of dots that form a triangle:

By adding another row of dots and counting all the dots we can 
find the next number of the sequence.

A Rule

We can make a "Rule" so we can calculate any triangular number.

First, rearrange the dots (and give each pattern a number n), like this:

Then double the number of dots, and form them into a rectangle:

The rectangles are n high and n+1 wideand xn is how many dots in the triangle (the value of the Triangular Number n)

And we get (remembering we doubled the dots):

2xn = n(n+1)xn = n(n+1)/2

Rule: xn = n(n+1)/2

Example: the 5th Triangular Number is

x5 = 5(5+1)/2 = 15

Example: the 60th is

x60 = 60(60+1)/2 = 1830

Wasn't it much easier to use the formula than to add up all those dots?

Answer 2

You add one more each time you need to find the next number in the pattern. To find the number after 36 you have to add 9. Then, to find the number after that, you will have to add 10.