Equilateral triangle

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[asy] pair A=(0,50),B=(43.301,-25),C=(-43.301,-25); draw(A--B--C--A); label("$60^{\circ}$",(-28,-16)); draw(anglemark(B,C,A,300)); label("$60^{\circ}$",(28,-16)); draw(anglemark(A,B,C,300)); label("$60^{\circ}$",(0,35)); draw(anglemark(C,A,B,300)); [/asy]

An equilateral triangle is a triangle in which all sides have equal length and all angles have equal measure. Since a triangle has a total of $180$ degrees, each angle of an equilateral triangle has $60$ degrees.

The equilateral triangle is the simplest regular polygon. It is also the only type of polygon that such that the equilateral polygon must also be the equiangular polygon.

One useful way to manipulate an equilateral triangle is to draw an altitude, which would divide it into two 30-60-90 right triangles. The altitude of the triangle is $\frac{s \sqrt{3}}{2}$. Using this, the area of an equilateral triangle can be found in terms of a side: $\frac{s^2\sqrt{3}}{4}$.

Notice that all equilateral triangles are similar (since all equilateral triangles have the same angle measurements, so, by AA similarity, all equilateral triangles are similar).