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IB DP Maths AA SL Study Notes

3.2.2 Co-Function Identities

Introduction to Co-Function Identities

Co-function identities are a special set of trigonometric identities that establish a connection between the values of sine, cosine, tangent, cotangent, secant, and cosecant for complementary angles. The most fundamental of these identities is: sin(90° - x) = cos(x)

This identity implies that the sine of an angle in a right triangle is the same as the cosine of its complementary angle.

Understanding the Geometry Behind the Identity

To truly grasp the essence of this identity, let's visualise it using a right triangle. Imagine a right triangle ABC with angle x at vertex A. The side opposite to this angle (BC) represents the sine of the angle, while the side adjacent (AC) represents the cosine of the complementary angle (90° - x).

Given the inherent properties of right triangles and the very definitions of sine and cosine, it's evident that: sin(x) = cos(90° - x)

This geometric relationship is consistent for all values of x and is the foundation of the co-function identity.

Diving Deeper: Variations of the Primary Identity

The primary co-function identity can be expanded to encompass other trigonometric functions. Here are the main variations:

1. Tangent and Cotangent: tan(90° - x) = cot(x) This relationship indicates that the tangent of an angle is the same as the cotangent of its complementary angle.

2. Secant and Cosecant: sec(90° - x) = csc(x) This identity connects the secant of an angle to the cosecant of its complementary angle.

Practical Applications of the Co-Function Identities

The co-function identities are not just theoretical constructs; they have a plethora of practical applications in various domains of mathematics and its allied fields.

Simplifying Expressions

One of the primary uses of co-function identities is in simplifying trigonometric expressions. For instance, if you come across an expression like sin(90° - x), you can instantly replace it with cos(x) using the primary co-function identity, making the expression more manageable.

Solving Trigonometric Equations

Equations involving trigonometric functions can often be tricky. However, co-function identities can be a lifesaver in such scenarios. They allow for the transformation of one trigonometric function into another, which can simplify the equation and facilitate its solution.

Real-World Applications: Physics and Engineering

In the realm of physics, co-function identities find their use in diverse problems related to wave motion, oscillations, and optics. Engineers, on the other hand, employ these identities in various applications, ranging from signal processing to control systems.

Delving into Examples

To solidify our understanding, let's explore some detailed examples that elucidate the application of the co-function identities.

Example 1: Simplifying an Expression

Question: Simplify the expression sin(90° - x) + cos(x).

Solution: Utilising the primary co-function identity, we can deduce that sin(90° - x) = cos(x). Thus, the expression can be rewritten as: cos(x) + cos(x) = 2cos(x)

Example 2: Solving a Trigonometric Equation

Question: Solve for x in the equation sin(90° - x) = 0.5.

Solution: Applying the primary co-function identity, the equation can be transformed to: cos(x) = 0.5 From this equation, we can deduce the possible values of x.

Key Takeaways

  • Co-function identities are a bridge that connects the trigonometric functions of complementary angles.
  • These identities have their roots in the properties of right triangles and the fundamental definitions of trigonometric functions.
  • Their versatility makes them indispensable tools in trigonometry, with applications ranging from simplifying expressions to solving complex equations and real-world problems in physics and engineering.


Co-function identities primarily relate the trigonometric functions of an angle to its complementary angle, which is specifically within the range of 0° to 90°. However, by using periodic properties of trigonometric functions and their behaviour in different quadrants, these identities can be extended and applied to angles greater than 90°. It's essential to have a clear understanding of the trigonometric functions' values in various quadrants to apply co-function identities effectively for larger angles.

Yes, co-function identities exist for all primary trigonometric functions. For secant and cosecant, the identity is: sec(90° - x) = csc(x). This means that the secant of an angle is equivalent to the cosecant of its complementary angle. Just like the identities for sine and cosine, this relationship is derived from the properties of right triangles and the definitions of the trigonometric functions.

Co-function identities are invaluable tools when it comes to simplifying trigonometric expressions. They allow for the conversion of one trigonometric function into another, based on the relationship between an angle and its complementary angle. For instance, an expression involving sin(90° - x) can be immediately simplified to cos(x) using the primary co-function identity. This ability to transform and simplify expressions makes mathematical calculations more straightforward and efficient.

Co-function identities find numerous applications in the real world, especially in the fields of physics and engineering. For instance, in wave mechanics, the relationship between sine and cosine functions, which are phase-shifted by 90°, can be understood using co-function identities. Similarly, in signal processing, these identities help in analysing phase differences between signals. Engineers often use these identities to simplify complex trigonometric expressions, making them more manageable for practical applications.

The term "co-function" is derived from the word "complementary". In trigonometry, two angles are said to be complementary if their sum is 90°. The co-function identities establish a relationship between the trigonometric functions of an angle and its complementary angle. For instance, the sine of an angle is related to the cosine of its complementary angle. This complementary nature of the relationships between different trigonometric functions is the reason behind the name "co-function" identities.

Practice Questions

Given that sin(θ) = 0.6, where 0° < θ < 90°, find the value of cos(90° - θ).

To solve this question, we can use the co-function identity: sin(θ) = cos(90° - θ). Given that sin(θ) = 0.6, it directly implies that cos(90° - θ) is also 0.6. Therefore, the value of cos(90° - θ) is 0.6.

If tan(α) = 3, where 0° < α < 45°, determine the value of cot(90° - α).

Using the co-function identity for tangent and cotangent, we have: tan(α) = cot(90° - α). Given that tan(α) = 3, it means that cot(90° - α) is the reciprocal of 3, which is 1/3. Thus, the value of cot(90° - α) is 1/3.

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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