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**Simplify an expression**

EXAMPLE 3 Simplify an expression Simplify the expression cos (x + π). cos (x + π) = cos x cos π – sin x sin π Sum formula for cosine = (cos x)(–1) – (sin x)(0) Evaluate. = – cos x Simplify.

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**Solve a trigonometric equation**

EXAMPLE 4 Solve a trigonometric equation Solve sin ( x + ) + sin ( x – ) = 1 for 0 ≤ x < 2π. π 3 Write equation. sin ( x + ) + sin ( x – ) π 3 = 1 Use formulas. sin x cos cos x sin sin x cos – cos x sin π 3 = 1 sin x cos x sin x – cos x 1 2 3 = 1 Evaluate. sin x = 1 Simplify. ANSWER In the interval 0 ≤ x <2π, the only solution is x = π 2

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EXAMPLE 5 Solve a multi-step problem Daylight Hours The number h of hours of daylight for Dallas, Texas, and Anchorage, Alaska, can be approximated by the equations below, where t is the time in days and t = 0 represents January 1. On which days of the year will the two cities have the same amount of daylight? Dallas: h1 Anchorage: h2 π t 182 = 2 sin ( – 1.35) π t 182 = –6cos ( )

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EXAMPLE 5 Solve a multi-step problem SOLUTION STEP 1 Solve the equation h1 = h2 for t. 2 sin ( – 1.35) π t 182 π t 182 = – 6 cos ( ) sin ( – 1.35) π t 182 π t 182 = – 3 cos ( ) sin ( ) cos 1.35 – cos ( ) sin 1.35 π t 182 π t 182 = – 3 cos ( ) sin ( ) (0.219) – cos ( ) (0.976) π t 182 π t 182 = – 3 cos ( ) 0.219 sin ( ) π t 182 π t 182 = – cos ( )

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EXAMPLE 5 Solve a multi-step problem π t 182 tan ( ) = – 9.242 π t 182 = tan –1 (– 9.242) + nπ π t 182 – nπ t – n STEP 2 Find the days within one year (365 days) for which Dallas and Anchorage will have the same amount of daylight. t – (1) 97, or on April 8 t – (2) 279, or on October 7

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GUIDED PRACTICE for Examples 3, 4, and 5 Simplify the expression. 6. sin (x + 2π) 8. tan (x – π) tan x ANSWER sin x ANSWER 7. cos (x – 2π) cos x ANSWER

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GUIDED PRACTICE for Examples 3, 4, and 5 π t 75 π t 75 9. Solve 6 cos ( ) + 5 = – 24 sin ( ) + 5 for 0 ≤ t < 2π. about 5.65 ANSWER

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Sum and Difference Formulas. WARM-UP The expressions sin (A + B) and cos (A + B) occur frequently enough in math that it is necessary to find expressions.

Sum and Difference Formulas. WARM-UP The expressions sin (A + B) and cos (A + B) occur frequently enough in math that it is necessary to find expressions.

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