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IB DP Maths AA SL Study Notes

3.1.1 Introduction to Radians

What is a Radian?

A radian is a standard unit of angular measurement. Adopted under the Système International d'Unités (SI), it's extensively utilised in mathematics, physics, and engineering disciplines. The definition of a radian is intrinsically tied to the properties of a circle. Specifically, one radian is the angle created at the centre of a circle by an arc whose length matches the radius of the circle. For more on how radians relate to arc lengths, see Arc Length.

To visualise, consider a circle with a radius 'r'. If you were to take a segment of the circle's circumference equivalent to the radius and measure the angle it creates at the circle's centre, that angle would be one radian. In terms of degrees, 1 radian is roughly equivalent to 57.295 degrees.

The Circle and Radians

The bond between radians and circles is profound. A circle, with its continuous curvature, has a circumference described by 2πr, where 'r' is its radius. When we discuss the angles within a circle in terms of radians:

  • A full circle encompasses an angle of 2π radians.
  • Half a circle, representing a straight angle, measures π radians.
  • A quarter of a circle, or a right angle, measures π/2 radians.

This intrinsic relationship between the circle's circumference and radians leads to several insightful observations:

  • The angle for a complete rotation or a full circle is 2π radians, equivalent to 360°.
  • Halfway around the circle, or 180°, is equivalent to π radians.

To understand how radians are used in calculating sector areas, check out Sector Area.

Conversion between Degrees and Radians

Switching between degrees and radians is a crucial skill, especially when tackling trigonometric problems. Here's a detailed breakdown of the conversion:

From Degrees to Radians

To convert an angle from degrees to radians, utilise the relationship that 180° is equivalent to π radians. The conversion formula is: Angle in radians = Angle in degrees x (π/180)

Example: Convert 90° to radians.

Using the formula: Angle in radians = 90 x (π/180) This gives π/2 radians. Therefore, 90° is equivalent to π/2 radians.

From Radians to Degrees

To convert from radians to degrees: Angle in degrees = Angle in radians x (180/π)

Example: Convert π/3 radians to degrees.

Using the formula: Angle in degrees = (π/3) x (180/π) This results in 60°. So, π/3 radians is equivalent to 60°.

For further reading on basic trigonometric equations involving radians, visit Basic Equations.

Practical Applications and Examples

Radians aren't just a theoretical concept; they have a myriad of practical applications, especially in physics, engineering, and computer graphics.

Example Question 1: A wheel with a 1-metre radius rotates such that a point on its edge travels an arc length of 1 metre. How many radians and degrees has the wheel turned?

Solution: Given the arc length equals the radius, the angle subtended is 1 radian. To find this angle in degrees: Angle in degrees = 1 x (180/π) This results in approximately 57.295°.

Thus, the wheel has turned through 1 radian or about 57.295°.

Example Question 2: A sector of a circle with an area of 5π square units and a 5-unit radius. What's the angle of the sector in radians?

Solution: The area of a sector is given by 0.5 x r2 x angle in radians. Using the data: 5π = 0.5 x 52 x angle in radians Solving for the angle gives 2 radians.

For more on how to find the equation of a tangent line involving radians, see Equation of a Tangent Line.

In essence, understanding radians is pivotal for anyone delving into maths, especially topics related to geometry and trigonometry. It offers a natural way to comprehend and measure angles, especially concerning the properties of circles. Whether you're diving into advanced maths or just embarking on your mathematical journey, a robust understanding of radians and their relationship with degrees will be invaluable. For visual learners, the Graphs of Sine can provide a clear depiction of these concepts.


The value of π is fundamental in the relationship between degrees and radians because it represents the ratio of a circle's circumference to its diameter. When we define angles in terms of radians, we are essentially relating the angle to portions of the circle's circumference. Since a full circle's circumference is 2π times its radius and a full circle also encompasses 360°, it naturally follows that 360° is equivalent to 2π radians. This relationship makes π the bridge between the two units of angular measure, and it's the reason why π frequently appears in formulas and conversions between degrees and radians.

Yes, besides degrees and radians, there are other units of angular measure, though they are less commonly used. One such unit is the gradian (or gon). In the gradian system, a right angle is 100 gradians, making a full circle 400 gradians. This system is sometimes used in surveying. Another unit is the turn, where a full circle is considered one turn. This unit is intuitive as it directly relates to the idea of completing rotations. However, in most mathematical, scientific, and engineering applications, degrees and radians remain the predominant units of angular measure due to their historical significance and practicality.

The unit circle in trigonometry is a circle with a radius of 1 unit, centred at the origin of a coordinate plane. Radians play a crucial role in the unit circle's definition and understanding. When we measure angles in radians on the unit circle, the angle's measure directly corresponds to the arc length subtended by that angle. For example, an angle of π/2 radians on the unit circle corresponds to an arc length of π/2 units. This direct relationship between angle measure and arc length in the unit circle simplifies the understanding and computation of trigonometric values, making it a foundational concept in trigonometry.

The radian is considered a "natural" unit of angular measure because it directly relates the angle measurement to the radius of a circle. When an angle is measured in radians, it represents the length of the arc subtended by that angle, divided by the radius of the circle. This relationship simplifies many mathematical expressions, especially in calculus and higher-level mathematics. For instance, when working with trigonometric functions in calculus, derivatives and integrals become more straightforward when angles are measured in radians. Moreover, many mathematical relationships and formulas, such as those in Fourier series or Taylor series expansions of trigonometric functions, are naturally expressed in terms of radians.

Radians play a pivotal role in understanding and describing circular motion in physics. When an object moves in a circular path, its angular displacement, angular velocity, and angular acceleration are often measured in radians. Using radians simplifies equations and relationships in circular motion. For instance, the linear velocity (v) of an object moving in a circle of radius (r) with an angular velocity (ω) is given by v = rω. Here, if ω is in radians per unit time, the equation directly gives the linear velocity without the need for conversion factors. Similarly, many formulas in rotational dynamics, such as torque and moment of inertia, are more straightforward and intuitive when angles are measured in radians.

Practice Questions

A circular pond has a diameter of 10 metres. A duck swims from one point on the edge of the pond to another point, covering an arc length of 8 metres. Calculate the angle, in radians, subtended by the arc at the centre of the pond.

To determine the angle in radians subtended by the arc at the centre, we can use the formula: Angle in radians = Arc length / Radius Given the diameter is 10 metres, the radius is half of that, which is 5 metres. Using the formula: Angle in radians = 8 metres / 5 metres = 1.6 radians. Thus, the angle subtended by the arc at the centre of the pond is 1.6 radians.

A wheel with a radius of 2 metres is rotating. If the wheel turns through an angle of π/4 radians, how long is the arc travelled by a point on the edge of the wheel?

To determine the arc length travelled by a point on the edge of the wheel, we can use the formula: Arc length = Radius x Angle in radians Given the radius is 2 metres and the angle is π/4 radians, using the formula: Arc length = 2 metres x π/4 = π/2 metres. Thus, the arc length travelled by a point on the edge of the wheel is π/2 metres, which is approximately 1.57 metres.

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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