2012 · Q62
In a triangle ABC, angle BCA = 60 degrees and AB^2 = BC^2 + CA^2 + X. What is X equal to?
Elementary MathematicsGeometry
- A(BC)(CA)
- B-(BC)(CA)
- C(AB)(BC)
- DZero
Explanation
By the cosine rule AB^2 = BC^2+CA^2-2(BC)(CA)cos60 = BC^2+CA^2-(BC)(CA), so X = -(BC)(CA).