Chapter 3 - Radian Measure and the Unit Circle - Section 3.3 The Unit Circle and Circular Functions - 3.3 Exercises - Page 123: 2

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$\cos 0$ is the x-coordinate of the point on the unit circle through which the terminal side of angle 0 passes. But as angle=0, terminal side and initial side of the angle coincides. Initial side is the positive x-axis. Positive x-axis meets the unit circle at the point (1,0) and therefore the x-coordinate=1=$\cos0$.