Use exact trigonometric values and related angles
Derive special-angle values from triangles and use a unit-circle position to determine signs and reference angles.
Section 1.5 · Trigonometry
Pure Mathematics 1 (Paper 1) · AS & A Level Mathematics 9709, 2026–2027. State angle units and find every solution in the supplied interval; general forms of solution are not required in section 1.5. Written practice is self-checked.
Before you begin
- Use right-triangle trigonometric ratios and Pythagoras.
- Convert special angles between degrees and radians.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is sin 30° exactly?
What you will learn
- Use exact sine, cosine and tangent values for 30°, 45° and 60°.
- Determine exact related-angle values in any quadrant, including negative angles and extra turns.
Derive the table instead of rounding it
A right isosceles triangle with shorter sides 1 and 1 has hypotenuse √2, so sin 45° = cos 45° = √2/2 and tan 45° = 1. Halving an equilateral triangle of side 2 gives a 30°–60°–90° triangle with sides 1, √3 and 2. Thus sin 30° = 1/2, cos 30° = √3/2, tan 30° = √3/3; sin 60° = √3/2, cos 60° = 1/2, tan 60° = √3.
The equivalent radian angles are π/6, π/4 and π/3. An exact answer retains fractions and surds: 0.866 is an approximation to √3/2. Rationalising 1/√3 gives √3/3; the two forms are equal. Use tan θ = sin θ/cos θ only where cosine is non-zero.
A unit circle supplies the sign
The unit-circle point at angle θ has coordinates (cos θ, sin θ). In quadrant I both are positive; in II cosine is negative and sine positive; in III both are negative; in IV cosine is positive and sine negative. Tangent takes the sign of their quotient. A reference angle is the acute angle to the x-axis; it provides the magnitude, while the quadrant determines the sign.
At 150° the reference angle is 30° and the point is in quadrant II, so cos 150° = −√3/2 and sin 150° = 1/2. At 225°, both coordinates are −√2/2 and tangent is 1. At 330°, tan is −√3/3. Reduce an angle by whole turns before locating its quadrant: sin 390° = sin 30° = 1/2. Negative angles rotate clockwise, so sin(−45°) = −√2/2.
Pause and explain
What is sin(5π/4) exactly?
Worked example
Find sin(2π/3), cos(2π/3) and tan(2π/3) exactly.
Show the worked solution
- 2π/3 = 120°, in quadrant II with reference angle 60°.
- Use the special-angle magnitudes and quadrant signs: sin = √3/2 and cos = −1/2.
- Divide sine by cosine to obtain tan = −√3.
Answer sin(2π/3) = √3/2; cos(2π/3) = −1/2; tan(2π/3) = −√3.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
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.
Find sin 120°, cos 120° and tan 120° exactly.
Give me a hint
Identify the reference angle and quadrant before applying the special-angle values.
Compare my reasoning
- 120° is in quadrant II and its reference angle is 180° − 120° = 60°.
- Sine is positive √3/2 and cosine is negative −1/2.
- Their quotient gives tangent −√3.
sin 120° = √3/2; cos 120° = −1/2; tan 120° = −√3.
Look for these in your work
- Select the correct reference angle.
- Apply separate sine and cosine signs before dividing.
Find sin(−π/4) and cos(7π/6) exactly.
Give me a hint
Locate a clockwise quarter-turn reference angle and a quadrant-III π/6 reference angle.
Compare my reasoning
- −π/4 is −45°, so its sine is negative with magnitude √2/2.
- 7π/6 = 210°, with reference angle π/6 in quadrant III.
- Cosine is negative there, giving −√3/2.
sin(−π/4) = −√2/2; cos(7π/6) = −√3/2.
Look for these in your work
- Handle negative rotation without losing its sign.
- Use the reference-angle cosine magnitude in the correct quadrant.
A point is 8 units from the origin at anticlockwise angle 5π/6 from the positive x-axis. Find its exact coordinates.
Give me a hint
Use (r cos θ, r sin θ) and check the quadrant and distance.
Compare my reasoning
- 5π/6 is 150°, in quadrant II, with cos = −√3/2 and sin = 1/2.
- Multiply both unit-circle coordinates by 8 to get (−4√3, 4).
- The squared distance is 48 + 16 = 64, confirming radius 8 and the expected negative x, positive y signs.
(−4√3, 4).
Look for these in your work
- Scale both unit-circle coordinates by the radius.
- Check the exact distance and quadrant signs.
Common mistakes
- A reference angle gives magnitude; it does not decide the sign.
- A rounded decimal is not an exact surd value.
- Tangent is undefined when cosine is zero.
What can you explain now?
Find cos 150° and tan 330° exactly.
Compare with the explanation
cos 150° = −√3/2; tan 330° = −√3/3.
Both have reference angle 30°, but quadrant II makes cosine negative and quadrant IV makes tangent negative.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Rebuild the 30°, 45° and 60° values from two triangles, then explain how to get a negative-angle or quadrant-III value without memorising another table.
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.