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**7-3 Sine and Cosine (and Tangent) Functions 7-4 Evaluating Sine and Cosine**

Identify a unit circle and describe its relationship to real numbers Evaluate Trigonometric functions using the unit circle Use domain and period to evaluate sine and cosine functions Use a calculator to evaluate trigonometric functions sin is an abbreviation for sine cos is an abbreviation for cosine tan is an abbreviation for tangent csc is an abbreviation for cosecant Sec is an abbreviation for secant Cot is an abbreviation for cotangent Homework: Page , #1, 3, 11, 13, 15, 17

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**In General The Unit Circle**

In your notes, please copy down the general ratios but keep in mind that for a unit circle r = 1. x P(x,y) r y Note that csc, sec, and cot are reciprocals of sin, cos, and tan. Also note that tan and sec are undefined when x = 0 and csc and cot are undefined when y = 0. In General The Unit Circle ππ ππ= π π¦ , yβ 0 ππ ππ= 1 π¦ , yβ 0 π πππ=π¦ sπππ= π π₯ , xβ 0 πππ π=π₯ sπππ= 1 π₯ , xβ 0 π‘πππ= π¦ π₯ , π₯β 0 πππ‘π= π₯ π¦ , yβ 0 π‘πππ= π¦ π₯ , π₯β 0 πππ‘π= π₯ π¦ , yβ 0

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**A few key points to write in your notebook: **

P(x,y) r x y A few key points to write in your notebook: P(x,y) can lie in any quadrant. Since the hypotenuse r, represents distance, the value of r is always positive. The equation x2 + y2 = r2 represents the equation of a circle with its center at the origin and a radius of length r. Hence, the equation of a unit circle is written x2 + y2 = 1. The trigonometric ratios still apply no matter what quadrant, but you will need to pay attention to the +/β sign of each.

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(β3,2) r β3 2 Example: If the terminal ray of an angle ο± in standard position passes through (β3, 2), find sin ο± and cos ο±. You try this one in your notebook: If the terminal ray of an angle ο± in standard position passes through (β3, β4), find sin ο± and cos ο±.

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**Example: If ο± is a fourth-quadrant angle and sin ο± = β5/13, find cos ο±.**

Example: If ο± is a second quadrant angle and cos ο± = β7/25, find sin ο±. x β5 13 Since ο± is in quadrant IV, the coordinate signs will be (+x, βy), therefore x = +12.

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**Determine the signs of sin ο± , cos ο± , and tan ο± according to quadrant**

Determine the signs of sin ο± , cos ο± , and tan ο± according to quadrant. Quadrant II is completed for you. Repeat the process for quadrants I, III, and IV. Hint: r is always positive; look at the red P coordinate to determine the sign of x and y. P(βx,y) r y r x y P(x,y) x P(βx, βy) r x y P(x, βy) r x y

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**Check your answers according to the chart below: **

All are positive in I. Only sine is positive in II. Only tangent is positive in III. Only cosine is positive in IV. y x All Sine Tangent Cosine

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**Find the reference angle.**

Let ο± be an angle in standard position. The reference angle ο‘ associated with ο± is the acute angle formed by the terminal side of ο± and the x-axis. y y P(βx,y) P(x,y) r r If necessary, find a coterminal angle between 0ο° and 360 ο° or 0 and 2Ο. Find the reference angle. Determine the sign by noting the quadrant. Evaluate and apply the sign. x x y P(x, βy) r x y x r P(βx, βy)

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**Example: Find the reference angle for ο± = 135ο°.**

You try it: Find the reference angle for ο± = 5ο°/3. You try it: Find the reference angle for ο± = 870ο°.

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**Give each of the following in terms of the cosine of a reference angle:**

Example: cos 160ο° The angle ο±=160ο° is in Quadrant II; cosine is negative in Quadrant II (refer back to All Students Take Calculus pneumonic). The reference angle in Quadrant II is as follows: ο‘=180 β ο± or ο‘=180 β 160 = 20. Therefore: cos 160ο° = βcos 20ο° You try some: cos 182ο° cos (β100ο°) cos 365ο° Try some sine problems now: Give each of the following in terms of the sine of a reference angle: sin 170ο° sin 330ο° sin (β15ο°) sin 400ο°

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**Can you complete this chart?**

60ο° 30ο° 45ο° 60ο° 30ο° 45ο°

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**Give the exact value in simplest radical form.**

Example: sin 225ο° Determine the sign: This angle is in Quadrant III where sine is negative. Find the reference angle for an angle in Quadrant III: ο‘ = ο± β 180ο° or ο‘ = 225ο° β 180ο° = 45ο°. Therefore: You try some: Give the exact value in simplest radical form: sin 45ο° sin 135ο° sin 225ο° cos (β30ο°) cos 330ο° sin 7ο°/6 cos ο°/4

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