What you need to know
A radian is the angle made at the centre of a circle by an arc the same length as the radius. A full turn is 2π radians, so π radians = 180°. To convert degrees to radians multiply by π180; to convert radians to degrees multiply by 180π. Radians are used in calculus and in the formulas arc length l = rθ and sector area A = [frac 1/2]r[sup 2]θ, which only work when θ is in radians.
| Angle | 0 (0°) | π6 (30°) | π4 (45°) | π3 (60°) | π2 (90°) |
|---|
| sin | 0 | 12 | 1√2 | √32 | 1 |
| cos | 1 | √32 | 1√2 | 12 | 0 |
| tan | 0 | 1√3 | 1 | √3 | undefined |
For angles beyond 90° use the unit circle. Measuring anticlockwise from the positive x-axis, the four quadrants are remembered by ASTC ("All Stations To Central"): in quadrant 1 All ratios are positive; in quadrant 2 only Sin is positive; in quadrant 3 only Tan; in quadrant 4 only Cos. To find an exact value: (1) decide the quadrant, (2) find the reference angle — the acute angle made with the x-axis (180° − θ, θ − 180° or 360° − θ), (3) take the exact value of the reference angle and attach the sign from ASTC.
Convert 150° to radians and 3π4 to degrees
- 150° × π180 = 150π180 = [frac 5π/6]
- 3π4 × 180π = 3 × 1804 = 135°
Find the exact values of sin 150°, cos 210° and tan 5π4
- sin 150°: quadrant 2 (sin positive). Reference angle 180° − 150° = 30°. sin 150° = sin 30° = [frac 1/2]
- cos 210°: quadrant 3 (cos negative). Reference angle 210° − 180° = 30°. cos 210° = −cos 30° = −[frac √3/2]
- tan 5π4: 5π4 = 225°, quadrant 3 (tan positive). Reference angle 5π4 − π = π4. tan 5π4 = tan π4 = 1
A sector has radius 6 cm and angle π3. Find the arc length and the area.
- Arc length: l = rθ = 6 × π3 = 2π cm ≈ 6.28 cm
- Area: A = 12r2θ = 12 × 36 × π3 = 6π cm[sup 2] ≈ 18.85 cm[sup 2]
Solve sin x = 12 for 0 ≤ x ≤ 2π
- sin is positive in quadrants 1 and 2.
- Reference angle: sin−1(12) = π6.
- Quadrant 1: x = π6. Quadrant 2: x = π − π6 = 5π6.
- Solutions: x = [frac π/6], [frac 5π/6]
Solve 2 cos x + √3 = 0 for 0 ≤ x ≤ 2π
- Rearrange: cos x = −√32.
- cos is negative in quadrants 2 and 3. Reference angle: cos−1(√32) = π6.
- Quadrant 2: x = π − π6 = 5π6. Quadrant 3: x = π + π6 = 7π6.
- Solutions: x = [frac 5π/6], [frac 7π/6]
State the amplitude and period of y = 3 sin 2x
- For y = a sin (nx) or y = a cos (nx): amplitude = |a| and period = 2πn.
- Amplitude = 3 (the graph oscillates between −3 and 3).
- Period = 2π2 = π (one full wave every π units, so there are two full waves between 0 and 2π).
Common mistake: a calculator in the wrong mode. If a question uses radians (any angle written with π, or an arc-length or calculus question), the calculator must be in radian mode. Test: sin π6 should give 0.5. Another common mistake: writing a decimal when an exact value is required. "Exact" means leave surds and π in the answer: sin 60° = √32, not 0.87. Also remember that ASTC gives the sign only — the size always comes from the reference angle.
Check yourself
- Convert 60° to radians.
- Convert 7π6 to degrees.
- Find the exact value of cos 120°.
- Find the exact value of sin 4π3.
- A sector has radius 10 cm and angle π5. Find the arc length.
- Solve tan x = 1 for 0 ≤ x ≤ 2π.
Answers: 1. π3 2. 210° 3. −12 (quadrant 2, reference angle 60°) 4. −√32 (quadrant 3, reference angle π3) 5. 10 × π5 = 2π cm 6. x = π4, 5π4