Trigonometry Functions of Angles

Fundamental notions:
1. Trigonometric Functions of Angles
2. Evaluating Trigonometric Functions at Any Angle
3. Trigonometric Identities
4. Areas of Triangles
The reason we're hung up on the "reference" angle is because we can deduce the values of sine and cosine from just the interval ${\left(0,\frac{\pi}{2}\right)}$ , using symmetry:

We have that
${\cos(-\theta)=\cos(\theta)}$ and ${\sin(-\theta)=-\sin(\theta)}$
(cosine is even, sine is odd).
Now let's try some exercises: pp. 459, #3, 7, 11, 13, 19, 23, 25, 39, 45, 55

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