Curriculum
General Mathematics — Second Secondary
Unit 1: Real Functions and Curve Sketching
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Unit 2: Exponents, Logarithms and Applications
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Unit 3: Limits
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Text lesson
The Cosine Rule
The Cosine Rule
Common mistakes
Common mistakes
Key points
Forgetting the term −2bc cos Athe rule is not just the sum of the squares of two sides. The cosine term must be subtracted.
Forgetting the square rootthe rule gives the square of the side, so take the square root at the end.
Confusing the opposite side with an adjacent sidethe side opposite the angle is the one that does not touch its vertex, and it is the one we calculate.
Sign of cosine for an obtuse anglecos is negative for an obtuse angle, so the subtracted term becomes an addition and the side becomes longer.
The Cosine Rule · 4 / 5