Radian Measure (Cambridge O Level Additional Maths)

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Radian Measure

What are radians?

  • Radians are an alternative to degrees for measuring angles
  • 1 radian is the angle in a sector of radius 1 and arc length 1
    • A circle with radius 1 is called a unit circle
  • Radians are normally quoted in terms of π
    •  radians = 360°
    • π radians = 180°
  • The symbol for radians is c but it is more usual to see rad
    • Often, when π is involved, no symbol is given as it is obvious it is in radians
    • Whilst it is okay to omit the symbol for radians, you should never omit the symbol for degrees
  • In the exam you should use radians unless otherwise indicated

Definition of a radian using a unit circle

definition of 1 radian on a circle of radius 1. The angle formed when r=1 and arc length is 1.

How do I convert between radians and degrees?

  • Use π c = 180° to convert between radians and degrees
    • To convert from radians to degrees multiply by 180 over straight pi
    • To convert from degrees to radians multiply by straight pi over 180
  • Some of the common conversions are:
    • 2 straight pi space to the power of c space equals space 360 space degree
    • straight pi to the power of space c end exponent space equals space 180 space degree
    • straight pi over 2 to the power of c space equals space 90 space degree
    • straight pi over 3 to the power of c space equals space 60 space degree
    • straight pi over 4 to the power of c space equals space 45 space degree
    • straight pi over 6 to the power of space c end exponent space equals space 30 space degree
  • It is a good idea to remember some of these and use them to work out other conversions

some common conversions of radians to degrees, using multiples and factors of 2pi=180

Examiner Tip

  • Sometimes an exam question will specify whether you should be using degrees or radians and sometimes it will not, if it doesn't it is expected that you will work in radians
  • If the question involves π then working in radians is useful as there will likely be opportunities where you can cancel out π
  • Make sure that your calculator is in the correct mode for the type of angle you are working with

Worked example

a)
Convert 43.8° to radians.

Divide by 180°.

fraction numerator 43.8 degree over denominator 180 degree end fraction equals 73 over 300

Multiply by π radians.

73 over 300 cross times straight pi

bold 43 bold. bold 8 bold degree bold equals bold 73 over bold 300 bold pi bold equals bold 0 bold. bold 764 bold space begin bold style stretchy left parenthesis 3 sf stretchy right parenthesis end style

b)
Convert fraction numerator 5 straight pi over denominator 4 end fractionto degrees.

Divide by π radians.

fraction numerator 5 straight pi over denominator 4 end fraction divided by straight pi equals 5 over 4

Multiply by 180°.

5 over 4 cross times 180 degree

fraction numerator bold 5 bold pi over denominator bold 4 end fraction space bold radians bold equals bold 225 bold degree

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Amber

Author: Amber

Expertise: Maths

Amber gained a first class degree in Mathematics & Meteorology from the University of Reading before training to become a teacher. She is passionate about teaching, having spent 8 years teaching GCSE and A Level Mathematics both in the UK and internationally. Amber loves creating bright and informative resources to help students reach their potential.