Using period as an aide to evaluate for cosine

The video tutorial explains how to evaluate the cosine of -9π/4 by finding a coterminal angle. The instructor demonstrates adding 2π to -9π/4 to obtain a smaller angle, -π/4, and explains the importance of common denominators in this process. The tutorial then uses the unit circle to determine the cosine of -π/4, highlighting the reflection of the angle in the fourth quadrant and identifying the cosine as the x-coordinate, which is sqrt(2)/2.

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