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6.4 Trigonometric Functions

Learn how to define trigonometric ratios in the coordinate plane and express them in terms of the unit circle. Find coterminal angles and reference angles to simplify calculations. Discover how to find exact values for trigonometric functions and understand the signs in different quadrants.

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6.4 Trigonometric Functions

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  1. 6.4 Trigonometric Functions Objectives: Define the Trigonometric Ratios in the coordinate plane. Define the Trigonometric functions in terms of the unit circle.

  2. Example #1 Coterminal Angles Coterminal angles share the same terminal side from standard position. Find a positive & negative angle coterminal with the given angle. A. 30° B. C.

  3. Trigonometric Ratios on the Coordinate Plane Regardless of the length of the radius, the trigonometric ratios remain the same.

  4. The Unit Circle Because the length of the radius doesn’t matter with respect to the trig ratios, the unit circle was developed to simplify calculations. Therefore, any point P on the unit circle has the coordinates (cost, sin t).

  5. Memorizing the Unit Circle

  6. Reference Angles Reference Angles are positive acute angles formed from the terminal side of θ and the x-axis. They are used to simplify finding exact values for trigonometric ratios anywhere on the unit circle.

  7. Example #2Find the Following Reference Angles -480° 290° 540° - 480° = 60° 360° - 290° = 70°

  8. Example #2Find the Following Reference Angles

  9. Finding Exact Values for Trig Functions (– , +) (+, +) Look at the sin(30°). The sine value is always the y-coordinate, so the exact value is ½. This angle is also a reference angle for 150°, 210°, 330°, etc. If you look at the various values around the unit circle for the angles above, you’ll notice that they all have ½ as the value, but change signs depending on the quadrant. Since (x, y)  (cosθ, sinθ), then the signs of the trig values will follow the signs of the x- and y-coordinates. (+ , –) (– , –) Steps to finding exact values: Find the reference angle. Find the exact value for the trig function of the reference angle. Translate the value to the correct quadrant changing the signs if necessary.

  10. Example #3Find the Exact Value for the Trig Functions sin 120° tan -405° Reference: Reference: The sine values stay positive in Quadrant II since they are the y-coordinates. Original angle is in Quadrant II. Original angle is in Quadrant IV. The tangent values become negative in Quadrant IV.

  11. Example #3Find the Exact Value for the Trig Functions sec csc Reference: Original angle is in Quadrant I. Since cosine is positive, so is secant. Reference: Original angle is in Quadrant II. Since sine is positive in Quadrant II, so is cosecant.

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