# Arcsin as an Integral/Lemma 2

## Lemma

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

- $\displaystyle \map {\arcsin_G} x = \int_0^x \frac {\d x} {\sqrt {1 - x^2} }$

## Proof

This result will be used in proving Derivative of Sine Function in the geometric case. So we can not use the same reasoning as Arcsin as an Integral/Lemma 1 because our logic would be circular.

Let $\theta$ be the length of the arc associated with the angle on the circle of radius $1$.

By definition of arcsine:

- $y = \sin \theta \iff \theta = \arcsin y$

We have that arc length is always positive.

For negative $y$, the $\arcsin$ function is defined as being the negative of the arc length.

This makes the $\arcsin$ function and the $\sin$ function odd, and puts us in line with mathematical convention:

Without this convention, the derivative of the $\sin$ function would not be continuous.

Now:

\(\text {(1)}: \quad\) | \(\displaystyle x^2 + y^2\) | \(=\) | \(\displaystyle 1\) | Equation of Circle | |||||||||

\(\displaystyle \dfrac {\d x} {\d y}\) | \(=\) | \(\displaystyle -\dfrac y x\) | Implicit Differentiation | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle -\dfrac y {\sqrt {1 - y^2} }\) | substituting for $x$ |

Then:

\(\displaystyle \arcsin_G y\) | \(=\) | \(\displaystyle \int_0^y \sqrt {1 + \paren {\dfrac {\d x} {\d y} }^2} \rd y\) | Definition of Arc Length | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \int_0^y \sqrt {1 + \paren {-\dfrac y x}^2}\) | substituting for $\dfrac {\d x} {\d y}$ | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \int_0^y \sqrt {1 + \dfrac {y^2} {x^2} } \rd y\) | |||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \int_0^y \sqrt {\dfrac {x^2} {x^2} + \dfrac {y^2} {x^2} } \rd y\) | rewriting $1$ to create common denominator | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \int_0^y \sqrt {\dfrac {x^2 + y^2} {x^2} } \rd y\) | combining terms with common denominator | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \int_0^y \sqrt {\dfrac 1 {x^2} } \rd y\) | Equation of Circle $(1)$ | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \int_0^y \dfrac 1 x \rd y\) | in Quadrant $\text I$ and Quadrant $\text {IV}$ | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \int_0^y \dfrac 1 {\sqrt {1 - y^2} } \rd y\) | substituting for $x$ in Quadrant $\text I$ and Quadrant $\text {IV}$ |

$\Box$