Definition
Isosceles triangle
An isosceles triangle has two or more sides of the same length.
What it means
Look for a matching pair of side lengths. The triangle may be wide, narrow, upside down, or tilted; turning it does not change those lengths. This card uses the inclusive meaning of “at least two”.
Remember it
Twin tick marks on two sides signal a matching pair of lengths.
Example
A triangle has side lengths 5 cm, 5 cm, and 6 cm. The two 5 cm sides match, so the triangle is isosceles.
Watch for
A triangle with side lengths 4 cm, 5 cm, and 6 cm has no equal pair, so it is scalene rather than isosceles.
Common mix-up
Because “at least two” includes three, an equilateral triangle also counts as isosceles under this definition.
Exact meaning
An isosceles triangle is a non-degenerate triangle in which at least two side lengths are equal.