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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 circles
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:

Two angles can be classified as complementary, supplementary or explementary depending on whether they add to 90°, 180° or 360°.

Read more about angles
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.

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

Read more about the volume of a cone ( interactive widget)
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 radius can be found in terms of the diameter, circumference and area.

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 radius
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:

Triangles can be classified by their 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°).

The interior angles of a triangle add up to 180°. This means a missing angle can be found if two are known.



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.

Polygons have interior angles. The sum of the interior angles of a polygon with n sides is:



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 area of a sector of a circle is:



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
Read more about the area of a sector of a circle
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 circles
Understanding ellipses
Understanding parallelograms
Understanding rectangles
Understanding squares
Understanding trapezoids
Understanding triangles

Angles

Understanding angles
Understanding 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 circle
Finding 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 cone
Finding the volume of a cube
Finding the volume of a cylinder
Finding the volume of a sphere

Circles (Basic)

Understanding the radius
Understanding 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 triangles
Understanding 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 polygons
Understanding 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 circle
Understanding the equation of a circle
Understanding the circle theorems
Finding the area of a sector
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.

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