Curriculum
Pure Mathematics — Second Secondary
Unit 1: Real Functions and Curve Sketching
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Unit 2: Exponents and Logarithms
0/12
Unit 3: Limits
0/6
Text lesson
The Cosine Rule
The Cosine Rule
Type of an angle from the sign of cosine
Type of an angle from the sign of cosine
- We use cos A = (b2 + c2 − a2) / (2bc) to decide the type of the angle:
| Sign of the numerator (b2 + c2 − a2) | Type of angle A |
|---|---|
| Positive | Acute |
| Zero | Right |
| Negative | Obtuse |
- In particular, the longest side is opposite the largest angle.
- If the square of the longest side is greater than the sum of the squares of the other two sides, the triangle has an obtuse angle.
The Cosine Rule · 2 / 5