# Geometry Index

Geometry is the study of space and shape. Modern mathematics approaches geometry from the view point of the Cartesian Coordinate System. The Cartesian Coordinate System has an origin and the position of of a point in space is measured by its distance from this origin.

## Notation

The notation for a symbol is a small symbol written in text, sometimes followed by three letters that correspond to a figure.

A point in the cartesian coordinate system is denoted by two numbers in parentheses separated by a comma. The first number represents the distance from origin in the x-direction and teh second number represents the distance from the origin in the y-direction.

A circle in text is denoted using the circle symbol with a dot in the center followed by a letter that corresponds to the center point of the circle.

Complementary angles can visually be denoted as two angles who sum to a perpendicular or square angle.

Congruent angles are denoted with tick marks across the angle.

A line is denoted by two letters representing the start and end of the line with a line over top.

Parallel lines are denoted by the parallel symbol placed betwen the notation of the two lines. A line is denoted by the start and end letter with a line over top.

A perpendicular angle is visually denoted by drawing a square at the vertex of the angle. The measured angle is equal to π/2 radians or 90°.

The symbol for two perpendicular lines is a horizontal line with another line drawn perpendicular to it.

A point in the polar coordinate system is denoted by two numbers in parentheses separated by a comma. The first number represents the radius r (distance from the origin) and an angle θ (the greek letter theta) relative to the origin.

Slope is denoted as the change in y over the change in x. The capital greek letter delta (Δ) is used to represent the change in a variable.

Supplementary angles can visually be denoted as two angles who sum to 180 degrees or PI degrees.

The symbol theta is often used as a variable to represent an angle in illustrations, functions, and equations.

A triangle is denoted using the triangle symbol followed by three letters that represent the points of the triangle.

## Formulas

The area of a circle is give by one-half multiplied by τ (tau) mutliplied by the radius of the circle squared.

The area of any parallelogram is equal to the base multiplied by the height.

The Area of a rectangle is given by its width multiplied by its height.

The area of a trapezoid is given by its height multiplied by the sum of its top length and bottom length divided by two.

The area of a triangle is given by one half multiplied by its width and height.

The circumference of a circle is given the constant τ (tau) multpilied by the radius of the circle, where τ = 2π.

The distance between two points, in one dimension, is given by the absolute value of the difference between the two values.

The distance between two points, in two dimensions, is given by solving pythagorean's theorem for the length the hypotenuse of the right triangle formed by the two points.

To find the midpoint between two points, average the two points x coordinate together to get the midpoint's x coordinate, then average the two points y coordinate together to get the midpoints y coordinate.

The volume of a cone is given by one third multiplied by PI, the radius of its base squared, and its height.

The volume of a cube is given by the length of the cube length raised to the third power.

The volume of a cylinder is equal to PI multiplied by its radius squared and its height.

To calculate the volume of a rectangular prism multiply its height, width, and length together.

The volume of a sphere is given by two-thirds multiplied by the circle constant τ (tau) multiplied by the radius cubed.

## Concepts

An acute angle is an angle that is smaller than 90 degrees or PI fourths.

An angle is defined as the amount of rotation between two rays. Angles are measured using degrees and radians. A full rotation in degrees is 360°. A full rotation in radians is approximately 6.283 radians or τ (tau) radians.

Area is the physical amount of two dimensional space that a shape takes up. Area is measured in square units.

The Cartesian Coordinate System describes space of one, two, and three dimensions. Each point in space is represented by its distance relative to the origin of the system.

Chebyshev are the roots to a special series of polynomial equations.

This page describes the basic elements and properties of a circle.

Geometric construction is a classic form of math that studies building forms and shapes using a compass and straight edge.

Degrees is a unit of measure for angles. A full rotation is equal to 360 degrees. In the cartesian coordinate system, degrees are measured starting from the rightmost edge of the circle.

The golden ratio is a number represented by the greek letter ϕ (phi). The value of ϕ is approximately 1.618 and is a naturally occurring number in nature. The golden ratio is often associated with the golden rectangle whose sides form a ratio equal to ϕ.

The golden rectangle is a rectangle whose width divided by height is equal to the golden number (approximately 1.618).

The inscribed angle theorem states that an inscribed angle in a circle is equal to one-half the central angle.

In math an interval represents the valid values between an upper limit and a lower limit.

An obtuse angle is an angle that is larger than 90 degrees or PI fourths.

A Perpendicular angle, sometimes also referred to as a square angle, is exactly 90 degrees or PI fourths.

A Perpendicular angle, sometimes also referred to as a square angle, is exactly 90 degrees or PI fourths.

The greek letter π (pi) is a naturally occurring number that is defined by any circle's circumference divided by its diameter.

A point represents a position in space. In modern mathematics, space is represented using the cartesian coordinate system where there is an origin and the position of a point in space is measured by its distance from the origin.

The Polar Coordinate System describes points in space using an angle and radius relative to the origin.

The pythagorean theorem equates the square of the sides of a right triangle together.

Radians are a unit that measure angle using the radius of a circle. One radian is equal to the amount of rotation required to travel the length of one radius along the circumference of the circle.

Slope is the concept of how much a function is changing with respect to x.

This page describes the basic elements and properties of a sphere.

The circle constant τ (tau) is a number approximately equal to 6.283. The number is defined as the length of a circle's circumference divided by the length of its radius.

A triangle is a three sided geometric shape. The shape forms a basis for the subject of trigonometry and is used throughout mathematics and programming.

There are six total trigonometric functions that relate to the geometry of the right-triangle sine, cosine, tangent, cosecant, secant, and cotangent. The functions take the angle of a right triangle as input and return a ratio of two of its sides.

Volume measures the amount of three dimensional space an object occupies.

## Examples

This example demonstrates how the general form of a plus b squared can be interpretted geometrically.

To calculate the area of a circle given the radius, multiply one half by the circle constant mutliplied by the radius squared.

To find the area of a rectangle given the width and height, multiply the width and height together.

How to divide an angle into two equal parts using a compass and straight edge.

How to bisect line with compass and straight edge.

To calculate the circumference of a circle multiply the radius of the circle by the circle constant tau approximately equal to 6.283.

To calculate the circumference of a circle multiply the radius by the constant τ (tau) to get the result.

To calculate the circumference of a circle multiply the radius by the constant τ (tau) to get the result.

This video shows how to construct an ellipse using two tacks, a string, and pencil.

This example demonstrates how to construct an equilateral triangle using a compass and ruler. An equilateral triangle is a triangle whose sides are of equal length.

How to build the golden rectangle using a compass and ruler. This geometric construction also demonstrates how to find the golden ratio.

This example demonstrates how to construct an hexagon using a straight edge and ruler. A hexagon is a polygon with six equal length sides.

This example demonstrates how to construct an pentagon using a straight edge and ruler. A hexagon is a polygon with five equal length sides.

How to construct a perpendicular line given a straight line.

To derive the difference of two angles identities, two right triangles are placed next to eachother so their angles sum together to be one angle and one triangle's angle is the difference of the sum and the other.

The double angle identities can be derived using the sum of two angle identities in combination with Pythagorean's identity.

The double angle identities can be derived using the inscribed angle theorem on the circle of radius one.

The half-angle identities can be derived from the double angle identities by transforming the angles using algebra and then solving for the half-angle expression.

To derive the equation for the law of sines, observe the shared perpendicular line by two angles. The third angle can be included by repeating the process.

The law of sines can be derived using a triangle inscribed on the perimeter of a circle. The proof uses the inscribed angle theorem.

To find the point where two lines intersect set the equations equal together and solve for the x-coordinate. Then substitute the solved for coordinate back into one of the equations to get the y-coordinate.

The sum of two angles addition formula can be derived using a quadrilateral inscribed on a circle of diameter 1.

To derive the sum of two angles identities, two right triangles are placed next to eachother so their angles sum together, then their proportions are related together.

To find the distance between two points in one dimension, substitue the x values of the points into the formula for the distance between two points.

To find the distance between two points in one dimension, substitue the x values of the points into the formula for the distance between two points.

To find the equation of a line given two points first calculate the slope of the line and then the y-intercept.

To find the midpoint between two points, average the two points x coordinate together to get the midpoint's x coordinate, then average the two points y coordinate together to get the midpoints y coordinate.

This example shows how the pythagorean theorem is true for a right triangle with sides length 3, 4, and 5.

To calculate the volume of the cone with a radius of 1 and height of 3 substitute the values into the volume of a cone formula.

To calculate the volume of the cone with a radius of 2 and height of 3 substitute the values into the value of a cone formula.

To calculate the volume of the cone with a radius of 1 and height of 3.

To calculate the volume of the cone with a radius of 1 and height of 3.

## Graphs

TODO

## Interactives

This interactive demonstrates the connection between the unit circle and the trigonometric functions sine, cosine and tangent.

This interactive demonstrates the connection between a right triangle of hypotenuse one and the graph of the cosine function. Click and drag either point to change the angle that describes the right triangle, or change the angle in the input box.

This interactive demonstrates the connection between every right triangle of hypotenuse one and the graph of the sine function. Click and drag either point to change the angle that describes the right triangle, or change the angle in the input box.

This interactive illustrates how an angle is defined by two rays. The point where the two meet is called the vertex of the angle.