Answer: cos A = 60/84.
- A cos A = 60/84
- B cos A = 5/7
- C cos A = 10/12
- D cos A = 60/60
Correct answer: A. cos A = 60/84
Explanation: cos A = (b² + c² - a²)/2bc = (36+49-25)/(2×6×7) = 60/84 = 5/7. A = cos⁻¹(5/7).
Unit circle with standard angles 0, 30, 45, 60, 90 degrees; at 45 degrees the point is (cos45, sin45) = (1/sqrt2, 1/sqrt2).
Concept context
Ratios, identities, and applications in triangles