sat suite question viewer
Point lies on a unit circle in the xy-plane and has coordinates . Point is the center of the circle and has coordinates . Point also lies on the circle and has coordinates , where is a constant. Which of the following could be the positive measure of angle , in radians?
Explanation
Choice D is correct. It's given that the circle is a unit circle, which means the circle has a radius of . It's also given that point is the center of the circle and has coordinates and that point lies on the circle and has coordinates . Since the radius of the circle is , the value of must be , as all other points with an x-coordinate of are a distance greater than away from point . Since and are points on the unit circle centered at , let side be the starting side of the angle and side be the terminal side of the angle. Since side is on the positive x-axis and side is on the negative x-axis, side is half of a rotation around the unit circle, or radians, away from side . Therefore, the positive measure of angle , in radians, is equal to plus an integer multiple of . In other words, the positive measure of angle , in radians, is an odd integer multiple of . Of the given choices, only is an odd integer multiple of .
Choice A is incorrect. This could be the positive measure of an angle where the starting side is and the terminal side contains the point , not .
Choice B is incorrect. This could be the positive measure of an angle where the starting side is and the terminal side contains the point , not .
Choice C is incorrect. This could be the positive measure of an angle where the starting side is and the terminal side contains the point , not .