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8 Applications of Trigonometry Copyright © 2009 Pearson Addison-Wesley

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**Applications of Trigonometry**

8 8.1 The Law of Sines 8.2 The Law of Cosines 8.3 Vectors, Operations, and the Dot Product 8.4 Applications of Vectors 8.5 Trigonometric (Polar) Form of Complex Numbers; Products and Quotients 8.6 De Moivre’s Theorem; Powers and Roots of Complex Numbers 8.7 Polar Equations and Graphs 8.8 Parametric Equations, Graphs, and Applications Copyright © 2009 Pearson Addison-Wesley

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**De Moivre’s Theorem; Powers and Roots of Complex Numbers**

8.6 De Moivre’s Theorem; Powers and Roots of Complex Numbers Powers of Complex Numbers (De Moivre’s Theorem) ▪ Roots of Complex Numbers Copyright © 2009 Pearson Addison-Wesley 1.1-3

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**is a complex number, then**

De Moivre’s Theorem is a complex number, then In compact form, this is written Copyright © 2009 Pearson Addison-Wesley 1.1-4

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**Remember the following:**

r= 𝑎 2 + 𝑏 2 , tan θ= 𝑏 𝑎 Copyright © 2009 Pearson Addison-Wesley 1.1-5

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**Find and express the result in rectangular form.**

Example 1 FINDING A POWER OF A COMPLEX NUMBER Find and express the result in rectangular form. First write in trigonometric form. Because x and y are both positive, θ is in quadrant I, so θ = 60°. Copyright © 2009 Pearson Addison-Wesley 1.1-6

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**Now apply De Moivre’s theorem.**

Example 1 FINDING A POWER OF A COMPLEX NUMBER (continued) Now apply De Moivre’s theorem. 480° and 120° are coterminal. Rectangular form Copyright © 2009 Pearson Addison-Wesley 1.1-7

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nth Root For a positive integer n, the complex number a + bi is an nth root of the complex number x + yi if Copyright © 2009 Pearson Addison-Wesley 1.1-8

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nth Root Theorem If n is any positive integer, r is a positive real number, and θ is in degrees, then the nonzero complex number r(cos θ + i sin θ) has exactly n distinct nth roots, given by where Copyright © 2009 Pearson Addison-Wesley 1.1-9

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**In the statement of the nth root theorem, if θ is in radians, then**

Note In the statement of the nth root theorem, if θ is in radians, then Copyright © 2009 Pearson Addison-Wesley

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**Find the two square roots of 4i. Write the roots in rectangular form.**

Example 2 FINDING COMPLEX ROOTS Find the two square roots of 4i. Write the roots in rectangular form. Write 4i in trigonometric form: The square roots have absolute value and argument Copyright © 2009 Pearson Addison-Wesley

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**Since there are two square roots, let k = 0 and 1.**

Example 2 FINDING COMPLEX ROOTS (continued) Since there are two square roots, let k = 0 and 1. Using these values for , the square roots are Copyright © 2009 Pearson Addison-Wesley

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**Example 2 FINDING COMPLEX ROOTS (continued)**

Copyright © 2009 Pearson Addison-Wesley

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**Find all fourth roots of Write the roots in rectangular form.**

Example 3 FINDING COMPLEX ROOTS Find all fourth roots of Write the roots in rectangular form. Write in trigonometric form: The fourth roots have absolute value and argument Copyright © 2009 Pearson Addison-Wesley

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**Since there are four roots, let k = 0, 1, 2, and 3.**

Example 3 FINDING COMPLEX ROOTS (continued) Since there are four roots, let k = 0, 1, 2, and 3. Using these values for α, the fourth roots are 2 cis 30°, 2 cis 120°, 2 cis 210°, and 2 cis 300°. Copyright © 2009 Pearson Addison-Wesley

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**Example 3 FINDING COMPLEX ROOTS (continued)**

Copyright © 2009 Pearson Addison-Wesley

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Example 3 FINDING COMPLEX ROOTS (continued) The graphs of the roots lie on a circle with center at the origin and radius 2. The roots are equally spaced about the circle, 90° apart. Copyright © 2009 Pearson Addison-Wesley

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**There is one real solution, 1, while there are five complex solutions.**

Example 4 SOLVING AN EQUATION BY FINDING COMPLEX ROOTS Find all complex number solutions of x5 – i = 0. Graph them as vectors in the complex plane. There is one real solution, 1, while there are five complex solutions. Write 1 in trigonometric form: Copyright © 2009 Pearson Addison-Wesley

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**The fifth roots have absolute value and argument**

Example 4 SOLVING AN EQUATION BY FINDING COMPLEX ROOTS (continued) The fifth roots have absolute value and argument Since there are five roots, let k = 0, 1, 2, 3, and 4. Solution set: {cis 0°, cis 72°, cis 144°, cis 216°, cis 288°} Copyright © 2009 Pearson Addison-Wesley

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Example 4 SOLVING AN EQUATION BY FINDING COMPLEX ROOTS (continued) The graphs of the roots lie on a unit circle. The roots are equally spaced about the circle, 72° apart. Copyright © 2009 Pearson Addison-Wesley

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**Copyright © 2009 Pearson Addison-Wesley 1.1-21**

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