Meaning & use
isosceles
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Meanings
adjectivebefore a nounmathematics
Describing a triangle in which two of the sides are the same length, so that two of the angles are also equal.
Grammar: Almost always used before the word triangle: an isosceles triangle.
Patterns
- an isosceles triangle
Draw an isosceles triangle with two sides of five centimeters.
In an isosceles triangle, the angles opposite the equal sides match.
The roof of the shed forms an isosceles shape when seen from the front.
adjectivebefore a nounmathematics
Describing a trapezoid whose two non-parallel sides are equal in length.
Grammar: Used before trapezoid: an isosceles trapezoid; in British English the shape is called a trapezium.
Patterns
- an isosceles trapezoid
An isosceles trapezoid has a line of symmetry down the middle.
The bridge support was built as an isosceles trapezoid.
Students proved that the base angles of an isosceles trapezoid are equal.
Common combinations
- isosceles triangle
- An isosceles triangle has one line of symmetry.
- isosceles trapezoid
- The window was cut in the shape of an isosceles trapezoid.
- base of an isosceles triangle
- Find the base of the isosceles triangle first.
- equal sides of an isosceles triangle
- Label the equal sides of the isosceles triangle with the same letter.
- right isosceles triangle
- A right isosceles triangle has angles of 90, 45 and 45 degrees.
Commonly confused: isosceles or equilateral? isosceles or scalene?
isosceles vs equilateral
An equilateral triangle has all three sides equal, while an isosceles triangle has only two guaranteed equal sides.
isosceles vs scalene
A scalene triangle has three sides of different lengths, while an isosceles triangle has two that match.
Practice
A triangle has sides of 6 cm, 6 cm and 9 cm. What kind of triangle is it?
Choose one answer.
Exactly two sides match, so the triangle is isosceles. An equilateral triangle needs all three sides equal, and a scalene triangle has no equal sides.
Make it your own
Write a sentence using “isosceles”, then check it and compare it with the examples above.