Define a radian and convert angle units accurately
Measure an angle by arc length divided by radius, convert exact angles and select the matching calculator mode.
Pure Mathematics 1 (Paper 1) · AS & A Level Mathematics 9709, 2026–2027. Use radians in arc and sector formulae. Written practice is self-checked.
Before you begin
- Use circumference 2πr and simplify fractions.
- Identify the angle at a circle's centre.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many radians make one full turn?
What you will learn
- Explain the definition of one radian.
- Convert degrees and radians, retaining exact multiples of π where possible.
A radian compares two lengths
One radian is the angle at the centre subtended by an arc whose length equals the radius. For an arc of length s on a circle of radius r > 0, its central angle is θ = s/r radians. The ratio is independent of circle size: scaling both lengths by the same factor preserves the angle. Radians do not mean a number of degrees with a different label.
A complete circumference has length 2πr, so one full turn is (2πr)/r = 2π radians = 360°. Half a turn is π radians = 180°. An arc of length 12 cm on a circle of radius 8 cm subtends 12/8 = 3/2 radians, about 85.9°. Use the radius, not the diameter, in this definition.
Convert the value before using it
Convert degrees to radians by multiplying by π/180. Convert radians to degrees by multiplying by 180/π. Thus 225° = 5π/4 radians and 7π/6 radians = 210°. Keep the exact multiple of π in an algebraic answer; for an angle without π, such as 1.2 radians, use a calculator for its degree approximation.
Check calculator mode before evaluating a trigonometric function. sin(π/6) in radian mode and sin(30°) in degree mode both equal 1/2. Typing π/6 in degree mode evaluates a different angle. Negative angles describe the opposite direction of rotation, and angles larger than a full turn are valid; the conversion factor does not change.
Pause and explain
Convert 150° to an exact radian value.
Worked example
Convert 225° to radians and 7π/6 radians to degrees.
Show the worked solution
- Use π radians = 180°, so 225° becomes 225π/180.
- Simplify the fraction to obtain 5π/4 radians.
- For the reverse conversion, (7π/6)(180/π) = 210°.
Answer 225° = 5π/4 radians; 7π/6 radians = 210°.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Convert 135° to radians and π/8 radians to degrees.
Give me a hint
Use the two reciprocal conversion factors and cancel π in the second calculation.
Compare my reasoning
- Write 135π/180 and simplify by dividing numerator and denominator by 45.
- This gives 3π/4 radians.
- For π/8 radians, (π/8)(180/π) = 22.5°.
135° = 3π/4 radians; π/8 radians = 22.5°.
Look for these in your work
- Use the correct conversion direction.
- Retain an exact radian answer and state the degree unit.
Convert −240° to radians and 11π/6 radians to degrees.
Give me a hint
A signed angle uses the same conversion rule; keep its sign.
Compare my reasoning
- Multiply −240 by π/180 to obtain −4π/3.
- Multiply 11π/6 by 180/π to obtain 330°.
- The negative sign indicates clockwise rotation under the usual positive anticlockwise convention.
−240° = −4π/3 radians; 11π/6 radians = 330°.
Look for these in your work
- Preserve the direction sign.
- Simplify exact multiples of π correctly.
A circular wheel of radius 10 cm turns through an arc length 7 cm. Find its angle in radians, then in degrees to one decimal place.
Give me a hint
The angle is arc length divided by radius; only the second answer needs approximation.
Compare my reasoning
- Use θ = 7/10 = 0.7 radians.
- Convert to degrees: 0.7 × 180/π ≈ 40.1070°.
- Round the degree result at the end to 40.1°; the arc length is not a straight-line chord.
0.7 radians; 40.1°.
Look for these in your work
- Use the defining ratio with the radius.
- Round only after converting the angle.
Common mistakes
- A radian is defined using an arc and the radius, not a chord or diameter.
- Do not replace θ by a degree value in a radian formula.
- Retain π until the final numerical approximation.
What can you explain now?
An arc is 12 cm long and its circle has radius 8 cm. Find the central angle in radians and degrees.
Compare with the explanation
3/2 radians, approximately 85.9°.
The defining ratio is 12/8. Multiply 3/2 by 180/π for the degree value.
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Make it stick
Explain why one full turn is 2π radians, then convert one positive and one negative angle without using a memorised decimal for π.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.