Internal Angle of Equilateral Triangle
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Theorem
The internal angles of an equilateral triangle measure $60^\circ$ or $\dfrac \pi 3$ radians.
Proof
By definition, an equilateral triangle is a regular polygon with $3$ sides.
From Internal Angles of Regular Polygon, the size $A$ of each internal angle of a regular $n$-gon is given by:
- $A = \dfrac {\paren {n - 2} 180^\circ} n$
Thus:
- $A = \dfrac {180^\circ} n = 60^\circ$
From Value of Degree in Radians:
- $1^\circ = \dfrac {\pi} {180^\circ} \mathrm {rad}$
and so:
- $A = 60^\circ \times \dfrac {\pi} {180^\circ} = \dfrac \pi 3 \mathrm {rad}$
$\blacksquare$
Sources
- 1986: David Wells: Curious and Interesting Numbers ... (previous) ... (next): $60$
- 1997: David Wells: Curious and Interesting Numbers (2nd ed.) ... (previous) ... (next): $60$