Why do we care so much about triangles? Why don’t we have something like “trigonometric functions” for other shapes, like rectangles, pentagons, etc?

Furthermore, why does trigonometry focus on right triangles? What about other kinds of triangles?

Discuss:

- The relationship between distance/direction and coordinate axes.
- Triangulation: any shape with straight sides can be divided into triangles.

### Trigonometry in Right Triangles

Defn. Suppose is in the open interval , i.e., for . Take any right triangle so that its acute angle at the vertex has degrees and so that is the hypotenuse. Then is the right angle, as shown:

By definition,

Why is cosine called *co*-sine? *Look at complementary angles in a right triangle.*

*What about angles bigger than or less than ?*

### Trigonometry in the Unit Circle

Defn. Suppose is any real number, and let be the image of the point after a clockwise rotation about the origin of degrees. Then the -coordinate of is and the -coordinate of is .

- Is a point on the unit circle? How do we know?
- How do we justify the move from right triangles to the unit circle? If is in the open interval , does the “right triangle” definition of sine and cosine agree with the “unit circle” definition?

## Expressions vs Equations, Equations vs Identities

What is the difference between an equation and an identity?

Definition. An **equation in one variable ** is a question that asks, when two expressions and are given, whether there is a number so that is equal to when , i.e., so that .

Definition. An **identity** is an equation which is true for *all* values of the variable (sometimes we allow a limited number of exceptions, for example values of for which one or both sides of the equation is undefined).

To **solve an equation** means to obtain *all* the solutions of the equations.

- This can be done by starting with the given equation and applying a series of algebra steps, each
*preserving equality*, until the solution or solutions are obtained.

What does it mean when we make a series of algebra steps, each preserving equality, in solving an equation? Is every such step reversible? *Examples*.

To **prove an identity**, we must establish that it is true for any value of .

- This can be done by starting with one side and applying a series of algebra steps, each
*preserving the value of the expression*, to obtain the other side. - It can also be done by starting with a known identity and applying a series of algebra steps, each
*preserving equality*, to obtain the desired identity. - Finally, one can also be start with the given identity and apply a series of algebra steps, as long as each step is
*reversible*

Example. Prove the identity

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