Answer:
C) 1/√5
Step-by-step explanation:
You only need to know that cosine values have a magnitude no greater than 1 and that the cosine values of 4th-quadrant angles are positive. These facts get you to the correct answer: 1/√5.
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If you draw the ray from the origin to the given point, and you draw a vertical line from the given point to the x-axis, the right triangle will have a hypotenuse (h) given by the Pythagorean theorem as ...
h^2 = 4^2 + (-8)^2 = 16 +64 = 80
h = √80 = 4√5 . . . . . take the square root
Then the cosine of the angle is the ratio of the "adjacent side" (the x-coordinate) to the hypotenuse:
cos = 4/(4√5) = 1/√5
