# Geometry (Free Mathematics Lessons and Tests)

# What Is Geometry?

Geometry is a branch mathematics concerned with the size, shape and relative position of figures, or the study of lines, angles, shapes and their properties.# Shapes

- A circle is a shape containing a set of points that are all the same distance from a given point, its center.

- An ellipse is a shape containing a set of points whose distance from a two fixed points, the foci, add up to a constant.

An ellipse looks like a flattened circle.

- A parallelogram is a four sided shape with opposite sides parallel.

- A rectangle is a four sided shape with four right angles.

- A square is a four sided shape with four equal sides and four right angles.

- A trapezoid is a four sided shape with one pair of opposite parallel sides.

- A triangle is a shape with three sides and three angles.

Read more about ellipses

Read more about parallelograms

Read more about rectangles

Read more about squares

Read more about trapezoids

Read more about triangles

# Angles

An angle is created by two rays that have a common end point, called the vertex. The angle is also a measure of the rotation between the two rays.Angles can be measured in degrees (°) or in radians.

There are different types of angles:

- An acute angle is an angle of less than 90°.

- A right angle is an angle of 90°.

- An obtuse angle is an angle greater than 90° and less than 180°.

- A straight angle
is an angle of 180°.

- A reflex angle is an angle greater then 180° and less than 360°.

- A full angle is an angle of 360°.

- Complementary angles are two angles which add up to 90°.

- Supplementary angles are two angles which add up to 180°.

- Explementary angles are two angles which add up to 360°.

Read more about degrees

Read more about radians

Read more about the types of angles

Read more about acute angles

Read more about right angles

Read more about obtuse angles

Read more about straight angles

Read more about reflex angles

Read more about full angle Read more about complementary angles

Read about supplementary angles

Read more about explementary angles

# Areas

Area is the space contained within a 2-dimensional shape.- The area of a circle of radius r is:

- The area of a circle of diameter d is:

- The area of an ellipse with semi-major axis a and semi-minor axis b is:

- The area of a parallelogram where b is the length of the base and h is the height is:

- The area of a rectangle where b is the length of the base and h is the height is:

- The area of a square where a is the length of the side is:

- The area of a trapezoid where b
_{1}and b_{2}are the lengths of the bases (parallel sides) and h is the height of the trapezoid is:

- The area of a triangle where b is the length of the base and h is the height is:

- The area of a triangle, using trigonometry, where a and b are lengths of two sides of the triangle and C is the angle between them is:

Read more about the area of a circle using the diameter ( interactive widget)

Read more about the area of an ellipse ( interactive widget)

Read more about the area of a parallelogram ( interactive widget)

Read more about the area of a rectangle ( interactive widget)

Read more about the area of a square ( interactive widget)

Read more about the area of a trapezoid ( interactive widget)

Read more about the area of a triangle ( interactive widget)

Read more about the area of a triangle using trigonometry

# Volumes

Volume is the space contained within a 3-dimensional shape.- The volume of a cone of height h and a circular base of radius r is:

- The volume of a cube where a is the length of the side is:

- The volume of a cylinder with radius r and height h is:

- The volume of a sphere of radius r is:

Read more about the volume of a cube ( interactive widget)

Read more about the volume of a cylinder

Read more about the volume of a sphere ( interactive widget)

# Circles (Basics)

- The center is the point the same distance from the points on the circle.

- The radius is the line segment from the center of the circle to any point on the circle.

- The diameter is the line segment that contains the centre of the circle and has its endpoints on the circle.

The diameter can be found in terms of the diameter, circumference and area.

- A chord is a line whose endpoints lie on the circle.

- The circumference is the distance around the circle.

The circumference of a circle with radius r and diameter d is:

- An arc is a portion of the circumference.

- A sector is a region bounded by two radii and the arc lying between the radii.

- A segment is a region, not containing the center, bounded by a chord and an arc lying between the chord's endpoints.

- A tangent is a line that touches the circle at one point.

Read more about the diameter

Read more about finding the radius from the diameter

Read more about finding the radius from the circumference

Read more about finding the radius from the area

Read more about finding the diameter from the radius

Read more about finding the diameter from the circumference

Read more about finding the diameter from the area

Read more about the circumference of a circle

# Triangles

There are different types of triangles.Triangles can be classified by how many sides and angles are equal:

- Equilateral triangles have
**3**equal side lengths and angles.

- Isosceles triangles have
**2**equal side lengths and angles.

- Scalene triangles have
**0**equal side lengths and angles.

- Acute triangles have all acute angles (less than 90°).

- Obtuse triangles have one angle that is
**obtuse**(more than 90°, less than 180°).

- Right triangles have one
**right**angle (90°).

Read more about the types of triangle

Read more about equilateral triangles

Read more about isosceles triangles

Read more about scalene triangles

Read more about right triangles

Read more about the interior angles of a triangle

Read more about finding the missing angle in a triangle

# Polygons

A polygon is a 2-dimensional shape with straight sides.- A 3 sided polygon is a triangle.

- A 4 sided polygon is a quadrilateral.

- A 5 sided polygon is a pentagon.

- A 6 sided polygon is a hexagon.

- A 7 sided polygon is a hexagon.

- An 8 sided polygon is an octagon.

- A 9 sided polygon is a nonagon.

- A 10 sided polygon is a decagon.

- A 12 sided polygon is a dodecagon.

A regular polygon has equal side lengths and angles. Each interior angle in a regular polygon is:

Polygons have exterior angles which add up to 180°. Each exterior angle of a regular polygon is:

An interior and exterior angle of a polygon will add to 180°.

Read more about polygons

Read more about the interior angles of a polygon

Read more about finding the sum of the interior angles of a polygon

Read more about finding the interior angle of a regular polygon

Read more about the exterior angles of a polygon

Read more about finding the exterior angle of a regular polygon

Read more about the sum of the interior and exterior angle of a polygon

# Circles (Advanced)

A circle of radius r centered at the origin has an equation:A circle of radius r centered at (a, b) has an equation:

There are many circle theorems which relate to lines and angles in a circle.

- The tangent makes 90° with the radius which it meets at the point at which it touches.

- Two radii form the two equal sides of an isosceles triangle.

- If a line cuts through a chord of the circle, such that it crosses it at 90° and cuts it in half, that line passes through the centre of the circle.

- The angle subtended by an arc at the center of a circle is twice the angle subtended by the same arc at the circumference.

- A triangle drawn from two ends of a diameter makes 90° at the circumference.

- All triangles drawn from a chord make the same angle at the circumference.

- A cyclic quadrilateral is a 4 sided shape where each corner touches the circle. Both pairs of opposite angles add up to 180°.

- Two tangents drawn from the same point outside of the circle are the same length and form two congruent right triangles.

- The angle between a tangent and a chord at the point of contact is equal to the angle in the alternate segment.

The length of an arc is:

Read more about the basic equation of a circle

Read more about the equation of a circle

Read more about the circle theorems

- the tangent meets the radius at 90 degrees
- two radii make an isosceles triangle
- the perpendicular bisector of a chord passes through center
- the angle at the center is twice that at the circumference
- the angle in a semicircle is 90 degrees
- angles in the same segment are equal
- opposite angles in a cyclic quadrilateral add up to 180 degrees
- tangents from the same point are equal
- the angle in the alternate segment is equal

Read more about the area of a sector of a circle (radians)

Read more about the length of an arc

Read more about the length of an arc (radians)

##### Curriculum

# Geometry Lessons

# Shapes

Understanding circlesUnderstanding ellipses

Understanding parallelograms

Understanding rectangles

Understanding squares

Understanding trapezoids

Understanding triangles

# Angles

Understanding anglesUnderstanding degrees

Understanding radians

Understanding the types of angles

Understanding acute angles

Understanding right angles

Understanding obtuse angles

Understanding straight angles

Understanding reflex angles

Understanding full angles

Understanding complementary angles

Understanding supplementary angles

Understanding explementary angles

# Areas

Finding the area of a circleFinding the area of a circle using the diameter

Finding the area of an ellipse

Finding the area of a parallelogram

Finding the area of a rectangle

Finding the area of a square

Finding the area of a trapezium

Finding the area of a triangle

Finding the area of a triangle using trigonometry

# Volumes

Finding the volume of a coneFinding the volume of a cube

Finding the volume of a cylinder

Finding the volume of a sphere

# Circles (Basic)

Understanding the radiusUnderstanding the diameter

Finding the circumference of a circle

Finding the radius from the diameter

Finding the radius from the circumference

Finding the radius from the area

Finding the diameter from the radius

Finding the diameter from the circumference

Finding the diameter from the area

# Triangles

Understanding the types of trianglesUnderstanding equilateral triangles

Understanding isosceles triangles

Understanding scalene triangles

Understanding right triangles

Understanding the interior angles of a triangle

Finding the missing angle in a triangle

# Polygons

Understanding polygonsUnderstanding the interior angles of a polygon

Finding the sum of the interior angles of a polygon

Finding the interior angle of a regular polygon

Understanding the exterior angles of a polygon

Finding the exterior angle of a regular polygon

Understanding the interior and exterior angle of a polygon add up to 180 degrees

# Circles (Advanced)

Understanding the basic equation of a circleUnderstanding the equation of a circle

Understanding the circle theorems

- a tangent meets a radius at 90 degrees
- two radii make an isosceles triangle
- the perpendicular bisector of a chord passes through the center
- the angle at the center is twice that at the circumference
- the angle in the semicircle is 90 degrees
- the angles in the same segment are equal
- opposite angles in a cyclic quadrilateral add up to 180 degrees
- tangents from the same point are equal
- the alternate segment theorem

Finding the area of a sector (in radians)

Finding the length of an arc

Finding the length of an arc (in radians)

##### Note

**WHAT'S IN A NAME?**

Geometry comes from combining the Greek words 'ge' ("earth") and 'metria' ("measurement").

Geometry means measuring the earth or land. This is because initially geometry would have been used practically to measure areas of fields and the lengths of roads.