Is 1981 a triangular number? (Is 1981 a triangle?)

Is 1981 a triangular number?

Here we will define and determine if 1981 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.

1981 is a triangular number if we can arrange all 1981 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 1981 for n in our formula, we get the following result:

√(8 × 1981) +1 ≈ 125.8928

125.8928 is not an integer (whole number), therefore the answer to "Is 1981 a triangular number?" is as follows:

1981 = Not a triangular number

Since 1981 is not a triangular number, you cannot use 1981 objects or dots to create a symetric equilateral triangle similar to the one pictured above on this page.

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