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Precalculus with Calculus Previews, 6th Edition by Dennis G. Zill, Jacqueline M. Dewar

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4.6  Sum and Difference Formulas

INTRODUCTION In this section we continue our examination of trigonometric identities. But this time we are going to develop only those identities that are of particular importance in courses in mathematics and science.

Sum and Difference Formulas The sum and difference formulas for the cosine and sine functions are identities that reduce cos(x1 + x2), cos(x1x2), sin(x1 + x2), and sin(x1x2) to expressions that involve cos x1, cos x2, sin x1, and sin x2. We will derive the formula for cos (x1x2) first, and then we will use that result to obtain the others.

For convenience, let us suppose that x1 and x2 represent angles measured in radians. As shown in FIGURE 4.6.1(a), let d denote the distance between ...

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