Definition:Triangle (Geometry)/Isosceles

Definition
An isosceles triangle is a triangle in which two sides are the same length and the third side is a different length.


 * IsoscelesTriangle.png

Equilateral Triangle
An equilateral triangle is a triangle in which all three sides are the same length:


 * EquilateralTriangle.png

Thus it can be seen that an equilateral triangle is a special case of an isosceles triangle.

Base
We have that the base of any general triangle can be specified as any of the three sides, usually whichever happens to be at the bottom of any diagram in whatever orientation that the triangle happens to be drawn.

However, the base of the isosceles triangle is specifially defined to be as the side which is a different length from the other two.

Base Angles
The two (equal) angles adjacent to the base are called the base angles.

Apex
The angle opposite the base is called the apex of the triangle.

Legs
The sides adjacent to the apex are called the legs of the triangle.

Linguistic Note
The word isosceles comes from the Greek: $\iota \sigma \omicron \sigma \kappa \epsilon \lambda \epsilon \varsigma$, that is: from iso meaning equal, and skelos meaning leg.

Thus an isosceles triangle is literally an equal-leg triangle.

It is pronounced eye-sos-ell-ease, that is, with the emphasis on the second syllable.

The word skeleton comes from the same linguistic root.