Trigonometry · SS 1
Basic trigonometric identities
Find one ratio when you know another.
- sin2 θ + cos2 θ = 1
- tan θ = sin θcos θ
What the letters mean
- θ
- any angle
Worked example
θ is acute and sin θ = 35. Find cos θ.
Step 1: Use sin2 θ + cos2 θ = 1
cos2 θ = 1 − 925 = 1625
Step 2: Square root (acute, so positive)
cos θ = 45 = 0.8
Final answer: cos θ = 45 = 0.8
See it
Play with this picture from the Trigonometric Ratios lesson.
60° + 48° + 72° = 180°
This is an acute-angled, scalene triangle. Drag any corner!
Put the three angles together and they make a straight line: 180°.
Your task: make a right-angled triangle
Drag a corner until one angle shows 90°. Tip: line two sides up with the grid lines.