Welcome to the fundamentals of trigonometry. This webbook introduces highschool and college level trigonometry through the geometry of the circle. The content is intended to bridge the gap between basic trigonometry and advanced trigonometry. While some exposure to trigonometry is useful, it is not a prerequisit of this book. This book is the second of a threepart series:
Trigonometry is a translational subject. It connects and is used by many different areas of math and other disciplines. The subject provides a medium and set of tools to approach concepts such as triangles, circles, waves, ellipsis and hyperbolas; ideas that play a formative role throughout math and physics.
"You think it's about triangles, but really it's about circles."
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Trigonometry traditionally focuses on the properties of triangles, especially that of the right triangle. The triangle is simple, versatile and appears everywhere. However, as we will see in this book, the geometry of the triangle and the circle are intimately related; Understanding the circle and why it is so important to trigonometry is vital to approach more complicated ideas in math and physics. You think it’s about triangles, but really it’s about circles.
This book approaches trigonometry primarily through the lens of the circle in the cartesian plane as illustrated in the figure above. The radian angle system, circle constant and trigonometric functions are defined using the geometry of the circle.
An angle is defined as the amount of rotation between two rays. The point where the two rays start is called the vertex. Angles are a fundemental building block of trigonometry as every triangle’s shape is formed by three angles.
By convention, in the Cartesian Plane, an angle is measured starting at the positive direction with positive rotation in the counterclockwise direction as shown in figure below. The variable (theta) is often used to represent the angle. Of course, in the real world, angles come in all different orientations. This convention is so the study of the angle can be formalized.
It is also convention to measure an angle starting from the upwards direction down, just as clocks and other devices are oriented. This is more common when an angle is measured starting from the axis in three dimensions.
There are two systems in math used to measure angles: degrees and radians. The degree system is rooted in the origins of trigonometry, time keeping and astronomy. The degree system has been used for hundreds of years, but as math has evolved, the radian system has replaced degrees as the preferred system^{[1]}. This is because, in short, the radian system has properties that lead to nice properties and elegance in later math.
A full rotation in the cartesian plane is animated below. Note, when referring to the “cartesian plane” it is implied that this is the standard “XY” coordinate plane.
The circle constant (tau) is a geometric constant defined by the circumference of a circle divided by its radius. The circle constant makes using radians more practical. Shown below is the definition of the circle constant in terms of any circle.
The significance of a circle in the Cartesian plane
Continuing the theme of looking at an angle in isolation, let’s introduce the right triangle.
The trigonometric functions are periodic wave functions that are used throughout math and physics. The three main functions are introduced using the geometry of the righttriangle as functions that take an angle as input and return the ratio of two sides of the corresponding right triangle:
The definitions of these functions are extended to all angles representing the domain of all real numbers using the geometry of the circle. This is shown in the figure and definitions below. Note, the geometry of the righttriangle is still very much there, but these definitions have some real advantages when thinking about the functions.
Given an angle in radians the sine function returns the ratio of over the radius . Geometrically, this ratio is equal to the component of the corresponding point on the unit circle. The plot of the sine function is shown below for angles from to (tau) radians.
Since the function returns a ratio, all possible output of the function can be visualized by constraining the hypotenuse to be of length , which forms the shape of the unit circle. This is shown in the interactive below.
Given an angle in radians the cosine function returns the ratio of the adjacent side over its hypotenuse. The plot of the cosine function is shown below for the domain to (tau) radians, where represents a full rotation. The numeric values for radians are shown on the top edge of the plot’s grid.
Since the function returns a ratio, all possible output of the function can be visualized by constraining the hypotenuse to be of length , which forms the shape of the unit circle.
Given an angle in radians the tangent function returns the tangent line to the point on the unit circle. Since the component of a point goes to zero at a quarter turn and threequarters turn, the function diverges at those angles.
The unit circle is a circle of radius one, placed at the center of the cartesian coordinate system visualizes the output of the trigonometric functions. Besides being a nice visualization for the trigonometric functions, the unit circle’s properties also make it useful in later mathematics.
The interactive below demonstrates the geometry of the unit circle and the output of the trigonometric functions.
The unit circle chart(s) are common angles and their corresponding pairs found on the unit circle. As a result, teachers often use the chart as a way to test student’s knowledge of the trigonometry of the unit circle. For example, the chart below shows the angles and values derived from dividing the unit circle into twelve equal parts.
Here is another chart that shows the angles and values from dividing the unit circle into eight equal parts. Really a unit circle chart is just a set of common or simple points found on the unit circle.
Since the angles on the charts are common, the points can be solved for using the properties of the special triangles and similar triangles as shown on this page. Of course, more generally, the charts can be
The trigonometric identities are a set of equations that relate the trigonometric functions together. This section discusses the trigonometric identities and how to derive them using the unit circle.
 How to find line perpendicular to another
 How to derive the trigonometric identities (geometry, algebra & trig, and complex plane)
 How to implement a shading algorithm in 3D using a triangle mesh
 Decompose velocity vector into individual components
 Rotate the direction of a vector by an angle
 Find the angle between two vectors
 How to model 2D rotation in a coordinate system (image rotation problem) (linear algebra or complex numbers)
 How to implement a simplified 2D “global” positioning system
 How to model our solar system
The circle constant τ (tau) is a geometric constant approximately equal to 6.283. The numeric value is defined as the length of any circle's circumference divided by the length of its radius.
The greek letter π (pi) is a geometric constant approximately equal to 3.1416. Its value is equal to the length of any circle's circumference divided by its diameter.
This book introduces the subject of trigonometry using the degree angle system and the geometry of the righttriangle. It contains basic trigonometry terminology, functions and identities.
This webbook covers the fundamentals of trigonometry using the radian angle system and the geometry of the circle. The definitions of the trigonometric functions are extended using the geometry of the circle. The applications of trigonometry are also discussed.
This webbook is a deep dive into trigonometry in the complex plane. Building upon the concepts in the previous book, the fundementals of trigonometry, this book explores the definition of trigonometric functions, Euler's formula and Fourier transformations.
Degrees are a unit of measure for angles. A full rotation is equal to 360 degrees. In the XY Cartesian Coordinate System, degrees are measured starting from the rightmost edge of the circle.
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.
There are six trigonometric functions that relate to the geometry of the righttriangle 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.
The sine function returns the sine of a number provided in radians. In geometric terms, the function returns the vertical component of the point formed by the angle on the unit circle.
The cosine function returns the cosine of an angle provided in radians. In geometric terms, the function returns the ratio of the righttriangle's adjacent side over its hypotenuse.
The unit circle chart shows the position of the points along the circle that are formed by dividing the circle into eight and twelve parts.

Radians Versus DegreesWumbo (internal)