Equilateral Triangle is Equiangular
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Theorem
Let $\triangle ABC$ be an equilateral triangle.
Then $\triangle ABC$ is also equiangular.
Proof
Let $\triangle ABC$ be an equilateral triangle.
By definition of equilateral triangle, any two of the sides of $\triangle ABC$ are equal.
Without loss of generality, let $AB = AC$.
Then by Isosceles Triangle has Two Equal Angles:
- $\angle ABC = \angle ACB$
As the choice of equal sides was arbitrary, it follows that every two of internal angles of $\triangle ABC$ are equal.
Hence all $3$ internal angles of $\triangle ABC$ are equal.
$\blacksquare$
Also see
- Equiangular Triangle is Equilateral, of which this is the converse.
Sources
- 1968: M.N. Aref and William Wernick: Problems & Solutions in Euclidean Geometry ... (previous) ... (next): Chapter $1$: Triangles and Polygons: Theorems and Corollaries $1.12$: Corollary $2$
- 1977: Gary Chartrand: Introductory Graph Theory ... (previous) ... (next): Appendix $\text{A}.5$: Theorems and Proofs: Problem Set $\text A.5$: $32$
- 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): equilateral polygon
- 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): polygon
- 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): equilateral polygon
- 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): polygon