Pre-Calculus 11 Notes

By: Gen L.

Lesson 3: Cosine Law

What is Cosine Law?

Similar to the triangle used for Sine Law (see lesson 2), The Cosine Law uses the same triangle, with a bit of a different formula.
With Cosine Law, you can have all three sides & find an angle OR have two sides and one angle, but no complete pair.
Formula:

Notes & Notation

• • For some sides & angle .
• Notation
  • for side, for angle.
    • : Cosine Length Determinant (Cosine Law)
    • : Cosine Angle Determinant (Cosine Law)

Example 1: Side ()

• Determine the length of .

1. Define Variables:
   • let

Example 1, cont.

5. • Square root both sides.

Example 2: Angle ()

• Determine the measure of .

1. Define Variables
   • let
4. • Inverse Cosine both sides.

Example 2, cont.

Next Lesson