Honors Geometry Advanced Trigonometry 4. Trigonometric Functions with Exact Values For most angles, the values of the sine, cosine, and tangent functions are nonrepeating decimals which must be approximated by rounding. For example, sin 40 = 0.642787609… This is usually rounded to 0.6428 in places such as the trig tables in the back of your Geometry book. ordinate represents the sine. For example, if an angle on the unit circle terminates at (-0.6, 0.8), its cosine is -0.6, and its sin is 0.8. The tangent is still y/x, so it would be 0.8/-0.6, or -4/3. However, some angles have exact values. All the quadrant angles (0, 90, 180, 270) and the special angles (30, 45, 60) have exact values for their sines, cosines, and tangents. Naturally, any angle which is coterminal with the angles mentioned above also has exact values for trig functions. For example, 450 and -180 have trig functions with exact values because they are co-terminal with 90 and 180. For example, a 180 angle terminates at (-1,0). So its cosine is -1, its sine is 0, and its tangent (0/-1) is 0. In addition, any angle for which a 30, 45, 60 angle is the reference angle has a sine, cosine, and tangent with exact values. For example, 135 has exact trig values because its reference angle is the special angle 45. We can use any size circle we wish in trigonometry, but the most convenient circle is the unit circle, which has a radius of one unit and a center at the origin. y y y r 1 x x x Cos θ = r 1 Sin θ = So, for any point on the unit circle, the x co-ordinate represents the cosine of the angle terminating there, and the y-co- Here you can see that the sine of 30 is 3 1 , the cosine is , and the tangent 2 2 3 3 3 1 1 is . ( = = ) 2 3 2 3 3 Honors Geometry Advanced Trigonometry 4. Trigonometric Functions with Exact Values Any angle whose measure is evenly divisible by 15 has exact values for sine, cosine, and tangent. Exercises Fill in – then memorize – the table. θ sine cosine tangent 0° 90° The circle above shows that the sine and 2 cosine of 45 each equal . The 2 tangent of 45 equals 1 (a very useful fact down the road). 180° 270° You could continue constructing triangles to show that sin 60 = cos 60 = ½ , and tan 60 = 3 , 2 30° 3. 45° Sine, cosine, and tangent of other angles Sin 630 By co-terminal angles, sin 630 = sin 270 = -1 Cos 240 The reference angle for 240 is 60. Cosine is negative in Quad III, so cos 240 = -cos 60 = - ½ Tan -225 By co-terminal angles, tan(-225) = tan 135 The reference angle for 135 is 45. Tangent is negative in Quad II, so tan(-225) = tan 135 = -tan 45 = -1 60° Give exact values for the quantity. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. sin(-90) cos 900 tan(-720) sin 810 cos(-270) tan 450 sin 135 cos 120 tan 330 sin 210 cos 315 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. tan 150 sin 150 cos 300 tan 210 sin 120 cos 135 tan 225 sin 660 cos(-45) tan -315