2022 · Q79
In a triangle ABC, angle A = 60°. What is AB² + AC² - BC² equal to?
Elementary MathematicsGeometry: Lines, Angles and Triangles
- AAB.AC
- BAB.BC
- CAC.BC
- D2AB.AC
Explanation
By the cosine rule, BC² = AB²+AC²−2·AB·AC·cos60° = AB²+AC²−AB·AC, so AB²+AC²−BC² = AB·AC. The trap options come from misremembering the cosine rule with the wrong included angle. The keyed option is A.