standard_description: Apply the general Multiplication Rule in a uniform probability model, P(A and B) = P(A)P(B|A) = P(B)P(A|B), and interpret the answer in terms of the model.
problem_activity: Task
Below are pictures of the first three  triangular numbers:
[IMG]
In general, the $n^{\text{th}}$ triangular number is the total number of dots in $n$ columns where the columns have $1, 2, 3, \ldots, n$ dots.
The following picture relates the first three triangular numbers to areas of rectangles:
[IMG]
Use this idea to calculate the seventh triangular number, $1 + 2 + 3 + 4 + 5+ 6+ 7$.
Calculate the hundredth triangular number, $1 + 2 + 3 + \cdots + 98 + 99 + 100$.
Find a formula for calculating the $n^{\text{th}}$ triangular number, $1 + 2 + \cdots + (n-1) + n.$


aligns: no