Trigonometric Identities: Exploring the Relationships in Angles

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What is the Pythagorean Identity?

$\sin^2\alpha + \cos^2\alpha = 1$

State the Reciprocal Identity for tangent.

$\frac{1}{\tan\alpha} = \cot\alpha$

What are the Co-function Identities for sine and cosine?

$\sin(90^\circ - \alpha) = \cos\alpha$ and $\cos(90^\circ - \alpha) = \sin\alpha$

Give the Angle Addition Formula for cosine.

$\cos(\alpha + \beta) = \cos\alpha\cos\beta - \sin\alpha\sin\beta$

What is the Double Angle Formula for sine?

$\sin(2\alpha) = 2\sin\alpha\cos\alpha$

Define the trigonometric function for sine of angle $\alpha$ in a right triangle.

The sine of angle $\alpha$ is defined as the ratio of the side opposite angle $\alpha$ to the hypotenuse.

What is the reciprocal function of the cosine of an angle $\alpha$ in a right triangle?

The reciprocal function of the cosine of angle $\alpha$ is the secant of angle $\alpha$.

Express the tangent function of an angle $\alpha$ in terms of the side opposite and the side adjacent to $\alpha$.

The tangent of angle $\alpha$ is equal to the ratio of the side opposite angle $\alpha$ to the side adjacent to $\alpha$.

What is the definition of the cosecant of an angle $\alpha$ in trigonometry?

The cosecant of angle $\alpha$ is defined as the reciprocal of the sine of angle $\alpha$.

How is the cotangent of an angle $\alpha$ related to the tangent of the same angle?

The cotangent of angle $\alpha$ is the reciprocal of the tangent of the same angle.

Study Notes

Trigonometric Identities: Unraveling the Relationships in Angles

Trigonometry, a discipline that studies the relationships between the sides and angles of triangles, is a fundamental tool in mathematics with countless applications in physics, engineering, and other fields. At the heart of trigonometry lie the trigonometric identities, which establish connections between the six primary trigonometric functions: sine, cosine, tangent, cosecant, secant, and cotangent.

Defining Trigonometric Functions

Let's imagine a right triangle with angles (\alpha), (\beta), and (\gamma), where (\gamma = 90^\circ). The side opposite (\alpha) is denoted as (opposite)(\alpha) (or (opp_\alpha)), the side adjacent to (\alpha) is (adjacent)(\alpha) (or (adj_\alpha)), and the hypotenuse, (hyp), is the longest side. The trigonometric functions are then defined as follows:

[ \sin\alpha = \frac{opp_\alpha}{hyp} ]

[ \cos\alpha = \frac{adj_\alpha}{hyp} ]

[ \tan\alpha = \frac{opp_\alpha}{adj_\alpha} ]

The cosecant, secant, and cotangent functions are the reciprocals of the sine, cosine, and tangent functions, respectively:

[ \csc\alpha = \frac{1}{\sin\alpha} ]

[ \sec\alpha = \frac{1}{\cos\alpha} ]

[ \cot\alpha = \frac{1}{\tan\alpha} ]

Trigonometric Identities

Trigonometric identities are mathematical equations that connect different trigonometric functions or the same function evaluated at different angles. They are essential for simplifying and solving trigonometric problems. Some of the most commonly used identities include:

1. Pythagorean Identity:

[ \sin^2\alpha + \cos^2\alpha = 1 ]

2. Reciprocal Identities:

[ \frac{1}{\sin\alpha} = \csc\alpha ]

[ \frac{1}{\cos\alpha} = \sec\alpha ]

[ \frac{1}{\tan\alpha} = \cot\alpha ]

3. Co-function Identities:

[ \sin(90^\circ - \alpha) = \cos\alpha ]

[ \cos(90^\circ - \alpha) = \sin\alpha ]

[ \tan(90^\circ - \alpha) = \frac{1}{\tan\alpha} ]

4. Angle Addition and Subtraction Formulas:

[ \sin(\alpha + \beta) = \sin\alpha\cos\beta + \cos\alpha\sin\beta ]

[ \sin(\alpha - \beta) = \sin\alpha\cos\beta - \cos\alpha\sin\beta ]

[ \cos(\alpha + \beta) = \cos\alpha\cos\beta - \sin\alpha\sin\beta ]

[ \cos(\alpha - \beta) = \cos\alpha\cos\beta + \sin\alpha\sin\beta ]

5. Double Angle Formulas:

[ \sin(2\alpha) = 2\sin\alpha\cos\alpha ]

[ \cos(2\alpha) = \cos^2\alpha - \sin^2\alpha = 2\cos^2\alpha - 1 = 1 - 2\sin^2\alpha ]

6. Trigonometric Substitution:

[ \sin^2\alpha = \frac{1 - \cos(2\alpha)}{2} ]

[ \cos^2\alpha = \frac{1 + \cos(2\alpha)}{2} ]

[ \sin^3\alpha = \frac{3 \sin\alpha - \sin(3\alpha)}{4} ]

[ \cos^3\alpha = \frac{3 \cos\alpha + \cos(3\alpha)}{4} ]

Uses of Trigonometric Identities

Trigonometric identities are applied in countless ways, including:

  1. Solving trigonometric equations.
  2. Resolving oblique triangles.
  3. Finding the values of trigonometric functions for specific angles.
  4. Converting expressions with one trigonometric function into expressions with another trigonometric function.
  5. Simplifying and rearranging trigonometric expressions.
  6. Deriving relationships between sides and angles in triangles.
  7. Solving problems in physics, engineering, and other fields.


Trigonometric identities are the cornerstone of trigonometry, providing the tools needed for working with angles and their associated trigonometric functions. By appropriately applying these identities, we can simplify and solve problems in a variety of fields, from math and physics to engineering and beyond. These identities not only serve as a foundation for the subject but also demonstrate the beautiful connections within mathematics.

Delve into the world of trigonometry and discover the intricate relationships between angles through trigonometric identities. Learn how these identities connect various trigonometric functions and how they are crucial for solving complex mathematical problems. Explore Pythagorean identities, co-function identities, angle addition formulas, and more.

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