Which of the following is cos(π/3)?

• A. sqrt(3)/2
• B. sqrt(2)/2
• C. 0
• D. 1/2

Answer: (D)

Cosine of an angle on the unit circle is the x-coordinate of the point where the angle’s terminal ray meets the circle. At angle π/3 (60°), the point is (1/2, √3/2), so the x-coordinate is 1/2. Therefore, cos(π/3) equals 1/2. In a 30-60-90 triangle, the cosine of the 60° angle is the adjacent side over the hypotenuse, which is 1/2. The other values shown correspond to other standard angles: √3/2 for π/6, √2/2 for π/4, and 0 for π/2.