In this lesson, we'll learn words for many different kinds of 2D shapes.
- polygon
- right triangle
- hypotenuse
- equilateral triangle
- isosceles triangle
- obtuse triangle
- acute triangle
- scalene triangle
- trapezoid
- parallelogram
- parallel
- rhombus
A large focus of geometry includes polygons. Polygons are closed 2D shapes with sides made of straight lines. For example, triangles, squares and rectangles are polygons. 3-sided and 4-sided polygons have lots of special names.
3-Sided Shapes
3-sided shapes are called triangles. There are different kinds of triangles. Examine the examples below.
- Right triangles have exactly one right angle (a 90° angle). The longest side of a right triangle (opposite the 90° angle) is called the hypotenuse.
- Equilateral triangles have three equal sides and three equal angles.
- Isosceles triangles have two equal sides and two equal angles.
- Obtuse triangles have one angle that is more than 90°.
- Acute triangles have three angles that are less than 90°.
- Scalene triangles have zero equal sides and angles.
4-Sided Shapes
Shapes with four sides are called trapezoids or parallelograms. There are many kinds of trapezoids. The light colored shapes in the box below (labeled "acute," "right," "obtuse," "isosceles" and "3-sides equal") are trapezoids. Trapezoids have exactly one pair of parallel sides. (Lines are parallel when they can continue infinitely in both directions and never intersect.)
The darker colored shapes (labeled "parallelogram," "rectangle," "rhombus" and "square") are actually all parallelograms. A parallelogram has exactly two pairs of parallel sides.
- A rectangle also has 4 right angles but sides of different lengths.
- A rhombus has all sides the same length but no right angles. Basically, it's shaped like a diamond.
- A square has all sides the same length and 4 right angles.