Explaining the Tan(A+B) Formula in Trigonometry

May 14, 2024

Trigonometry is a branch of mathematics that deals with the study of angles, triangles, and the relationships between their sides and angles. One of the fundamental concepts in trigonometry is the addition formula for tangent, known as the Tan(A+B) formula. This formula allows us to express the tangent of the sum of two angles in terms of the tangents of the individual angles. Understanding and applying this formula is crucial for solving trigonometric equations and simplifying expressions involving trigonometric functions. In this article, we will delve into the Tan(A+B) formula, explore its derivation, and see how it can be used to solve problems in trigonometry.

Introduction to the Tan(A+B) Formula

The Tan(A+B) formula is given by:

$$\tan(A+B) = \frac{\tan A + \tan B}{1 – \tan A \cdot \tan B}$$

where A and B are the angles whose sum is being considered.

Derivation of the Tan(A+B) Formula

To derive the Tan(A+B) formula, we start with the tangent addition formula:

$$\tan(A+B) = \frac{\sin(A+B)}{\cos(A+B)}$$

Using the angle sum identities for sine and cosine:

$$\sin(A+B) = \sin A \cdot \cos B + \cos A \cdot \sin B$$

$$\cos(A+B) = \cos A \cdot \cos B – \sin A \cdot \sin B$$

Substitute these into the tangent addition formula:

$$\tan(A+B) = \frac{\sin A \cdot \cos B + \cos A \cdot \sin B}{\cos A \cdot \cos B – \sin A \cdot \sin B}$$

Now, divide both the numerator and the denominator by (cos A * cos B):

$$\tan(A+B) = \frac{\frac{\sin A}{\cos A} + \frac{\sin B}{\cos B}}{1 – \frac{\sin A \cdot \sin B}{\cos A \cdot \cos B}}$$

Recognizing that sin A/cos A = tan A and sin B/cos B = tan B, we obtain:

$$\tan(A+B) = \frac{\tan A + \tan B}{1 – \tan A \cdot \tan B}$$

This is the Tan(A+B) formula, which allows us to find the tangent of the sum of two angles in terms of the tangents of the individual angles.

Applications of the Tan(A+B) Formula

The Tan(A+B) formula finds applications in various areas of trigonometry and calculus. Here are some common scenarios where this formula is used:

  1. Trigonometric Identities: The Tan(A+B) formula is essential for proving various trigonometric identities involving tangent functions.

  2. Solving Trigonometric Equations: When solving trigonometric equations involving the sum of angles, the Tan(A+B) formula can be used to simplify expressions and solve for unknown variables.

  3. Trigonometric Integrals: In calculus, the Tan(A+B) formula is valuable for integrating functions involving tangent functions.

  4. Geometry Problems: The Tan(A+B) formula is applied in geometry problems where angles need to be combined or compared.

Example Problems Using the Tan(A+B) Formula

Let’s solve a couple of example problems to illustrate how the Tan(A+B) formula is applied in practice:

Example 1

Problem: Find the value of tan(75°) using the Tan(A+B) formula if tan(30°) = 1/√3 and tan(45°) = 1.


Given that tan(30°) = 1/√3 and tan(45°) = 1, we can apply the Tan(A+B) formula with A = 45° and B = 30°:

tan(75°) = (tan(45°) + tan(30°)) / (1 – tan(45°) * tan(30°))

Substitute the values:

tan(75°) = (1 + 1/√3) / (1 – 1/√3)

Simplify the expression:

tan(75°) = (√3 + 1) / (√3 – 1)

Rationalize the denominator:

tan(75°) = ((√3 + 1) / (√3 – 1)) * ((√3 + 1) / (√3 + 1))

tan(75°) = (3 + 2√3 + 1) / (3 – 1)

tan(75°) = (4 + 2√3) / 2

tan(75°) = 2 + √3

Therefore, tan(75°) = 2 + √3.

Example 2

Problem: If tan(θ) = 3 and tan(φ) = 4, find the value of tan(θ + φ).


Given tan(θ) = 3 and tan(φ) = 4, we can apply the Tan(A+B) formula with A = θ and B = φ:

tan(θ + φ) = (3 + 4) / (1 – 3 * 4)

tan(θ + φ) = 7 / (1 – 12)

tan(θ + φ) = 7 / -11

Therefore, tan(θ + φ) = -7/11.

FAQs about the Tan(A+B) Formula

  1. What is the Tan(A+B) formula used for?
  2. The Tan(A+B) formula is used to find the tangent of the sum of two angles in terms of the tangents of the individual angles.

  3. How is the Tan(A+B) formula derived?

  4. The formula is derived by applying angle sum identities for sine and cosine and simplifying the resulting expression.

  5. In what scenarios can the Tan(A+B) formula be applied?

  6. The formula is commonly used in trigonometric identities, solving equations, calculus integrals, and geometry problems.

  7. Can the Tan(A+B) formula be extended to more than two angles?

  8. Yes, the formula can be extended to find the tangent of the sum of multiple angles by applying the formula iteratively.

  9. What if one of the angles has a tangent of infinity or is undefined?

  10. In such cases, the Tan(A+B) formula may not be directly applicable, and alternative methods or approaches may be needed for solving the problem.

In conclusion, the Tan(A+B) formula is a powerful tool in trigonometry that enables us to simplify and compute the tangent of the sum of two angles efficiently. By understanding the derivation of the formula and practicing its application through example problems, students can enhance their proficiency in trigonometric calculations and problem-solving.

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