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The angle difference formula for cosine is explored.
Students begin by exploring a geometric representation of angle difference as they review unit circle concepts. The angle difference formula for cosine is applied to angles other than those often committed to memory through use of the unit circle. Students test their accuracy through the manipulation of a unit circle with a moveable point and displayed coordinates.
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An optional extension for students is the derivation of the angle difference and sum formulas for cosine. Diagrams similar from Problem 1 are used along with the Law of Cosines and the distance formula to obtain these two useful trigonometric identities.
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