Problem 1 - Discrete Mathematics with Applications

Question

Compute the 36th triangular number. (It is the so-called beastly number.)

Step-by-Step Solution

Answer

The 36th triangular number, also known as the beastly number, is 666. This is calculated using the formula \(\frac{n(n+1)}{2}\) with n = 36, as follows: \(\frac{36(36+1)}{2} = \frac{36(37)}{2} = \frac{1332}{2} = 666\).

Step 1: Write down the formula for the nth triangular number.

We will use the formula \(\frac{n(n+1)}{2}\) to find the 36th triangular number.

Step 2: Substitute n with 36 in the formula.

We need to substitute n with 36 in the formula, so it becomes: \[\frac{36(36+1)}{2}\]

Step 3: Simplify the equation.

Now, simplify the equation by performing the operations inside the parentheses first: \[\frac{36(37)}{2}\]

Step 4: Calculate the result.

Finally, multiply 36 by 37, and divide the result by 2: \[\frac{1332}{2}\] The result is 666. So, the 36th triangular number (the beastly number) is 666.

Key Concepts

Number Theory and Triangular Numbers

Number theory is a fascinating branch of mathematics that delves into the mysterious properties and patterns of numbers. It explores the characteristics and relationships between various sets of numbers such as integers, prime numbers, and special numbers, including triangular numbers.

Triangular numbers belong to a specific set of numbers that can form equilateral triangles. When you arrange objects like dots into a triangle, adding consecutive layers of dots each time, the total count of dots up to any given layer is a triangular number.

For example, if you have a triangle with 3 rows of dots:

• The first row has 1 dot,
• The second row has 2 dots,
• The third row has 3 dots.

The total is 6 dots, making 6 the third triangular number.

Interestingly, each triangular number can also be represented as the sum of a sequence of natural numbers starting from 1. This idea connects seamlessly with the broader realm of number theory, where finding patterns and deriving formulas to understand these numbers is key.

Sequence Formulas for Triangular Numbers

Understanding sequence formulas is crucial for calculating any term within a sequence, like triangular numbers. Triangular numbers present a beautiful sequence where each term corresponds to a triangular shape formed by arranging objects.

The formula that defines the nth triangular number is: \[T_n = \frac{n(n+1)}{2}\] This equation translates the visual idea of a triangular arrangement into a mathematical expression that quickly finds any term in the sequence.

Here's how it works:

• Start with 'n', which represents the term number in the sequence.
  For instance, if you want the 36th triangular number, use n = 36.
• Substitute 'n' into the formula: \[T_{36} = \frac{36(36+1)}{2}\]
• Calculate inside the parentheses first, multiply 36 by 37, and then divide by 2 to find the 36th triangular number, which results in 666.

This formula is a powerful tool in understanding and computing the mesmerizing sequence of triangular numbers.

Arithmetic Sequences and Triangular Numbers

An arithmetic sequence is a collection of numbers where each term differs from the previous one by a constant amount. Triangular numbers form a specific sequence, though they are not strictly arithmetic in the classic sense, they have a relation to arithmetic progressions.

When you examine the numbers making up each triangular number, they can be viewed as the sum of successive arithmetic increments.

• The initial terms are: 1, 3, 6, 10, ... where each number represents a triangular number.
• The differences between consecutive terms form an arithmetic sequence: 2, 3, 4, ...

Understanding this pattern is crucial—each step in forming a triangular number involves adding the number of terms we have added so far, following the arithmetic progression.

By recognizing this relationship, you can see how triangular numbers encapsulate a beautiful blend of geometry and arithmetic, linking visual patterns with algebraic formulas and enriching the broader study of arithmetic sequences.