Is 129 a triangular number? (Is 129 a triangle?)

Is 129 a triangular number?

Here we will define and determine if 129 is a triangular number, also known as a triangle number. First, note that an equilateral triangle is a triangle where the length of the three sides are the same, and the three internal angles of the triangle are 60 degrees. An equilateral triangle can look like this:

Take notice of the symmetry of the dots inside the triangle above. Each side of the triangle has the same number of dots, and one dot is added to each row of dots.

129 is a triangular number if we can arrange all 129 dots or objects in perfect symmetry to make an equilateral triangle. To determine if a number (n) is a triangular number, the following equation must be true:

√(8 × n) +1 = Integer

When we enter 129 for n in our formula, we get the following result:

√(8 × 129) +1 ≈ 32.1403

32.1403 is not an integer (whole number), therefore the answer to "Is 129 a triangular number?" is as follows:

129 = Not a triangular number

Since 129 is not a triangular number, you cannot use 129 objects or dots to create a symetric equilateral triangle similar to the one pictured above on this page.

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