中等MATH-TRIG-004
In a triangle with sides 5, 5 and 6, what is the cosine of the angle opposite the side of length 6?
A. 11/25
B. 18/25
C. 7/25✓ 答案
D. −7/25
解說
cos A = (5^2 + 5^2 − 6^2) / (2 × 5 × 5) = (50 − 36) / 50 = 14/50 = 7/25. Subtracting the wrong side produces 18/25. The cosine is positive because the angle opposite 6 is acute.