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IB DP Maths AA SL Study Notes

3.2.3 Double and Half Angle Identities

Delving Deeper into Double and Half Angle Identities

At the heart of trigonometry lie the sine, cosine, and tangent functions. These functions, when paired with the double and half-angle identities, provide a powerful toolkit for simplifying complex trigonometric expressions and solving intricate problems. For a better understanding of how these functions are graphed, see graphs of sine.

Double Angle Identities

The double angle identities are pivotal in expressing the trigonometric functions of a doubled angle in terms of the original angle. These identities are:

1. Sine Double Angle: The sine of a double angle can be expressed as a product of the sine and cosine of the original angle. Specifically, sin(2x) equates to 2sin(x)cos(x). This identity is particularly useful when we encounter trigonometric expressions that require simplification or when solving trigonometric equations.

2. Cosine Double Angle: The cosine of a double angle has three primary expressions. These are:

  • cos(2x) is equivalent to cos2(x) - sin2(x)
  • Alternatively, cos(2x) can be expressed as 2cos2(x) - 1
  • Lastly, cos(2x) can also be represented as 1 - 2sin2(x)

3. Tangent Double Angle: The tangent of a double angle is a ratio of the sine and cosine functions. Specifically, tan(2x) is equal to (2tan(x))/(1 - tan2(x)). To explore more on how tangent is graphed, visit graphs of tangent.

Half Angle Identities

The half-angle identities, on the other hand, provide a relationship between the trigonometric functions of an angle and half of that angle. These identities are:

1. Sine Half Angle: The sine of half an angle can be expressed in terms of the cosine of the original angle. Specifically, sin(x/2) is equal to ±√[(1 - cos(x))/2].

2. Cosine Half Angle: Similarly, the cosine of half an angle can be expressed in terms of the cosine of the original angle. Specifically, cos(x/2) equates to ±√[(1 + cos(x))/2].

3. Tangent Half Angle: The tangent of half an angle has a unique expression that relates it to the cosine of the original angle. Specifically, tan(x/2) is equal to ±√[(1 - cos(x))/(1 + cos(x))].

For more examples of using these identities in solving equations, see basic equations.

Real-World Applications

Beyond the classroom, the double and half-angle identities find numerous applications:

1. Physics: These identities play a crucial role in wave mechanics, where they help in analysing the phase differences between waves. They are also pivotal in understanding oscillations and vibrations.

2. Engineering: Engineers, especially those working in signal processing and control systems, often turn to these identities to simplify and analyse signals and systems.

3. Computer Graphics: In the realm of computer graphics, these identities help in rotations and transformations, ensuring that images and animations appear smooth and realistic. For more on how trigonometric concepts like arc length are applied, visit arc length.

4. Astronomy: Astronomers use these identities to calculate the positions of celestial bodies and understand their movements.

In addition, these identities are often used in the study of rational functions to simplify expressions and solve equations.

Deep Dive with Examples

To truly grasp the power and utility of the double and half-angle identities, let's delve into some detailed examples.

Example 1: Expressing in Terms of Single Angles

Question: Given that sin(x) = 1/2 and cos(x) = √3/2, express sin(2x) in terms of sin(x) and cos(x).

Solution: Using the double angle identity for sine, sin(2x) is equal to 2sin(x)cos(x). Plugging in the given values, sin(2x) becomes 2(1/2)(√3/2), which simplifies to √3/2.

Example 2: Finding Values Using Half Angle Identities

Question: If cos(x) = 4/5, determine the value of sin(x/2).

Solution: Using the half-angle identity for sine, sin(x/2) is equal to √[(1 - cos(x))/2]. Inserting the given value, sin(x/2) becomes √[(1 - 4/5)/2], which simplifies to √(1/10) or √(0.1). For further practice with trigonometric equations, refer to basic equations.


The unit circle is a fundamental concept in trigonometry, representing trigonometric functions geometrically. Double and half angle identities can be visualised and understood better using the unit circle. For instance, the cosine of a double angle can be visualised as the x-coordinate of a point on the unit circle corresponding to that angle. Similarly, the sine of a half angle can be related to the y-coordinate of a point halfway between the origin and a point on the unit circle. By using the unit circle, one can gain a deeper geometric insight into these identities, making them more intuitive and easier to remember.

Absolutely. Double and half angle identities find applications in various real-world scenarios, especially in physics and engineering. For instance, in the study of oscillations and waveforms, these identities help in expressing a sum of two oscillations as a product, making it easier to analyse. They're also used in electrical engineering, particularly in the analysis of alternating current circuits. Furthermore, in computer graphics, these identities are employed to perform rotations and transformations. Essentially, any scenario where wave patterns, oscillations, or rotations are involved, the double and half angle identities can play a crucial role in simplifying and solving problems.

Yes, there are several other identities in trigonometry that are closely related to double and half angle identities. Some of these include the product-to-sum identities and sum-to-product identities. These identities express products of trigonometric functions in terms of sums and differences, and vice versa. They can be derived using the double angle identities and the compound angle identities. Another set of related identities are the power-reduction identities, which express powers of trigonometric functions in terms of multiple angles. All these identities, together with the double and half angle identities, form a comprehensive toolkit in trigonometry, aiding in simplification, evaluation, and problem-solving.

Yes, the double angle identities can be derived from the compound angle identities. The compound angle identities express the sine, cosine, and tangent of the sum or difference of two angles in terms of the sines and cosines of those angles. By setting the two angles to be equal, we essentially get the double angle identities. For example, using the compound angle identity for cosine, cos(A + B) = cos(A)cos(B) - sin(A)sin(B), and setting A = B, we get the double angle identity for cosine: cos(2A) = cos2(A) - sin2(A).

Double and half angle identities are fundamental in trigonometry because they provide a bridge between the trigonometric values of an angle and its double or half. This connection is particularly useful in simplifying complex trigonometric expressions, solving trigonometric equations, and integrating trigonometric functions. Additionally, these identities can be used to derive other trigonometric identities and to express trigonometric functions in different forms. For instance, they can help in expressing a trigonometric function in terms of another function, making certain calculations more straightforward. Their importance is also evident in real-world applications, such as in physics and engineering, where they help in solving problems related to waveforms and oscillations.

Practice Questions

Given the double angle identity for cosine, cos(2x) = 2cos^2(x) - 1, find the value of x when cos(2x) = 1/2.

To solve for x when cos(2x) = 1/2, we can equate the given double angle identity to the provided value. Using the identity cos(2x) = 2cos2(x) - 1, we set it equal to 1/2: 2cos2(x) - 1 = 1/2 On solving for cos2(x), we get: cos2(x) = 3/4 Taking the square root on both sides, we get two possible values for cos(x), which are sqrt(3)/2 and -sqrt(3)/2. The general solutions for x corresponding to these values are x = pi/6 + 2pi n and x = -pi/6 + 2pi n respectively, where n is an integer.

Using the half angle identity for sine, express sin(x/2) in terms of cos(x).

The half-angle identity for sine is given by: sin(x/2) = ± sqrt(1 - cos(x))/2 This identity allows us to express the sine of half an angle in terms of the cosine of the original angle. The positive or negative sign is determined based on the quadrant in which x/2 lies. For instance, if x is in the first quadrant, then x/2 will also be in the first quadrant, and the sine value will be positive. On the other hand, if x is in the third quadrant, x/2 will be in the second quadrant, making the sine value negative.

Dr Rahil Sachak-Patwa avatar
Written by: Dr Rahil Sachak-Patwa
Oxford University - PhD Mathematics

Rahil spent ten years working as private tutor, teaching students for GCSEs, A-Levels, and university admissions. During his PhD he published papers on modelling infectious disease epidemics and was a tutor to undergraduate and masters students for mathematics courses.

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