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GB-33-05-001 · Cosine rule

In triangle ABC, AB = 25 cm, AC = 6 cm and angle BAC = 61°. Calculate BC to 1 decimal place.

Cosine Rule ("Cosine Sausage!" — Finding Side) Booklet p. 36
Method
1) Circle pair and enclosed angle A
2) Sub into a² = b² + c² − 2bc cos(A)
3) Square root to find a
Worked Example (GB-33-05-001)
Booklet p. 36 — "Cosine Sausage!" Enclosed angle AA between two sides: a2=b2+c2−2bccos⁡(A)a^2 = b^2 + c^2 - 2bc\cos(A)
1) BC2=252+62−2(25)(6)cos⁡61∘BC^2=25^2+6^2-2(25)(6)\cos 61^\circ
2) Take the positive square root.
3) 22.7 cm
22.7 cm