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Angle Addition Formulas in Trigonometry (Sine, Cosine, Tangent, and Cotangent)

Angle addition formulas are fundamental trigonometric identities that allow the calculation of sine, cosine, tangent, and cotangent of the sum of two angles. These formulas play a crucial role in simplifying expressions, solving trigonometric equations, and applying trigonometry in physics and engineering. This article provides a complete explanation of all angle addition relations involving α and β.

Angle addition formulasTrigonometric identitiesSine and cosine

~2 min read • Updated Feb 4, 2026

1. What Are Angle Addition Formulas?


In trigonometry, angle addition formulas are identities used to calculate the trigonometric functions of the sum of two angles such as α and β.


These formulas are especially useful when the direct value of α + β is unknown, but the individual trigonometric values of each angle are given.


2. Sine Angle Addition Formula


The angle addition formula for sine is defined as follows:


sin(α + β) = sin α cos β + cos α sin β

This formula shows that the sine of the sum of two angles equals the sum of the cross products of sine and cosine.


3. Cosine Angle Addition Formula


The angle addition formula for cosine is expressed as:


cos(α + β) = cos α cos β − sin α sin β

The key difference between this formula and the sine formula is the minus sign between the two terms.


4. Tangent Angle Addition Formula


The tangent of the sum of two angles is calculated using the following identity:


tan(α + β) = (tan α + tan β) / (1 − tan α tan β)

It is important to ensure that the denominator does not equal zero; otherwise, tangent is undefined.


5. Cotangent Angle Addition Formula


The angle addition formula for cotangent is given by:


cot(α + β) = (cot α cot β − 1) / (cot α + cot β)

Although used less frequently, this formula is very useful in solving certain trigonometric equations.


6. Important Notes About Angle Addition Formulas


  • These formulas are valid for all angle values
  • They form the foundation of advanced trigonometric identities
  • They are essential for simplifying expressions and solving equations

7. Applications of Angle Addition Formulas


Angle addition formulas are widely used in mathematics, physics (such as wave motion and oscillations), engineering, and computer graphics.


Conclusion


Mastering the angle addition formulas for sine, cosine, tangent, and cotangent is a fundamental requirement in trigonometry. These identities provide powerful tools for analyzing and solving complex mathematical problems.


Written & researched by Dr. Shahin Siami

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