To find the cosine of the angle formed by rotating a ray on the positive x-axis by -450°, we can follow these steps:
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Determine the equivalent angle within the standard range: Angles are often expressed in a range from 0° to 360°. To get an equivalent angle for -450°, we can add 360° until we find a positive angle:
\[ -450° + 360° = -90° \] \[ -90° + 360° = 270° \]
So, -450° is equivalent to 270°.
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Find the cosine of the equivalent angle: The cosine of 270° can be determined using the unit circle:
\[ \cos(270°) = 0 \]
Thus, the cosine of the angle formed by the rotation of the ray is \( \boxed{0} \).