# Equivalence of Definitions of Sine of Angle

## Contents

## Theorem

Let $\theta$ be an angle.

The following definitions of the concept of **sine of $\theta$** are equivalent:

### Definition from Triangle

In the above right triangle, we are concerned about the angle $\theta$.

The **sine** of $\angle \theta$ is defined as being $\dfrac {\text{Opposite}} {\text{Hypotenuse}}$.

### Definition from Circle

Consider a unit circle $C$ whose center is at the origin of a cartesian coordinate plane.

Let $P = \left({x, y}\right)$ be the point on $C$ in the first quadrant such that $\theta$ is the angle made by $OP$ with the $x$-axis.

Let $AP$ be the perpendicular from $P$ to the $x$-axis.

Then the **sine** of $\theta$ is defined as the length of $AP$.

### Definition from Real Numbers

The real function $\sin: \R \to \R$ is defined as:

\(\displaystyle \sin x\) | \(=\) | \(\displaystyle \sum_{n \mathop = 0}^\infty \paren {-1}^n \frac {x^{2 n + 1} } {\paren {2 n + 1}!}\) | |||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle x - \frac {x^3} {3!} + \frac {x^5} {5!} - \cdots\) |

## Proof

### Definition from Triangle implies Definition from Circle

Let $\sin \theta$ be defined as $\dfrac {\text{Opposite}} {\text{Hypotenuse}}$ in a right triangle.

Consider the triangle $\triangle OAP$.

By construction, $\angle OAP$ is a right angle.

Thus:

\(\displaystyle \sin \theta\) | \(=\) | \(\displaystyle \frac {AP} {OP}\) | |||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \frac {AP} 1\) | as $OP$ is the radius of the unit circle | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle AP\) |

That is:

- $\sin \theta = AP$

$\Box$

### Definition from Circle implies Definition from Triangle

Let $\sin \theta$ be defined as the length of $AP$ in the triangle $\triangle OAP$.

Compare $\triangle OAP$ with $\triangle ABC$ in the diagram above.

We have that:

- $\angle CAB = \angle POA = \theta$
- $\angle ABC = \angle OAP$ which is a right angle

Therefore by Triangles with Two Equal Angles are Similar it follows that $\triangle OAP$ and $\triangle ABC$ are similar.

By definition of similarity:

\(\displaystyle \frac {\text{Opposite} } {\text{Hypotenuse} }\) | \(=\) | \(\displaystyle \frac {BC} {AC}\) | by definition | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \frac {AP} {OP}\) | by definition of similarity | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle AP\) | as $OP$ is the radius of the unit circle | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \sin \theta\) | by definition |

That is:

- $\dfrac {\text{Opposite} } {\text{Hypotenuse} } = \sin \theta$

$\blacksquare$

### Definition from Circle equivalent to Definition from Real Numbers

Let, $\sin_G$ be the Geometric Sine from Definition:Sine/Definition from Circle. $\arcsin_G$ is the inverse of this function.

Let $sin_A$ be the analytic sine function for real numbers, the one defined by Definition:Sine/Real Numbers. $\arcsin_A$ is the inverse of this function.

We know from Arcsin as an Integral that $\arcsin_A$ and $\arcsin_G$ are the same function.

$x=\sin_A\left({\theta}\right) \iff \arcsin_A\left(x\right)=\theta \iff \arcsin_G\left(x\right)=\theta \iff x=\sin_G\left({\theta}\right)$

$\blacksquare$