Introduction to Trigonometry

What this lesson covers

Introduction to Trigonometry is a 11-part Class 9 Mathematics lesson. Each part makes one point and then hands over to the next, so it can be read in a sitting or picked up again at the part you stopped on. It defines the idea in plain words before using it, works through an example step by step, breaks the topic into its parts and compares them, shows where it is used outside the textbook, names the mistakes that cost marks in an exam, ends with a recap of what to remember.

How it begins

Trigonometry relates the angles of a right-angled triangle to the ratios of its sides. The sides are named relative to the angle you are using: Hypotenuse, opposite the right angle. Always the longest, and it never changes. Opposite: across from the angle θ. Adjacent: next to θ, and not the hypotenuse. Critical: opposite and adjacent swap if you work from the other acute angle.

The 11 parts

1. Ratios in a right-angled triangle
2. The three ratios
3. Choosing which ratio to use
4. Finding a side
5. Finding an angle
6. Exact values worth memorising
7. Angles of elevation and depression
8. Why trigonometry exists
9. Where marks are lost
10. What to remember
11. Watch one worked through