5.2 Contents

1. Proof of $π$ 🔧

2. The Natural Number $e$ 🔧

3. The Golden Ratio $φ$ 🔧

4. The Plastic Ratio 🔧

5.2.1 Proof of $π$ 🔧

In regular polygons, as the number of sides → ∞, the perimeter approaches a circumference, wherein $π$ can be manually calculated.

5.2.2 The Natural Number $e$ 🔧

Continually compounded growth with 100% (1) return at a continuous rate yields a limit at $e$ $$e=\lim_{x→∞} {\bigg( 1+\frac{1}{x} \bigg)}^x=\lim_{x→0} {(1+x)}^{1/x}≈2.7 \medspace 1828 \medspace 1828 \medspace 459$$

$e$ Limit

$$\lim_{x→0}\frac{e^x-1}{x}=1$$

Natural Logarithm

The inverse function of $e^x$ is $\log_e x$, represented as $\ln(x)$

Limits of Natural Exponents & Logarithms

$$\lim_{x→\pm ∞}e^{\pm x}=∞\qquad\lim_{x→\pm ∞}e^{\mp x}=0$$

Proof of $e$ Limit

$$e=\lim_{x→0}{(1+x)}^{1/x}$$

5.2.3 The Golden Ratio $φ$ 🔧

Definition

Two quantities are in the golden ratio if their ratio is the same as the ratio of their sum to the larger of the two quantities $$\frac{x+a}{x}=\frac{x}{a}$$

Proof of Ratios

Given the ratio equality, substitute 1 for $a$ $$\frac{x+1}{x}=x$$ Multiply by $x$ $$x+1=x^2$$ Rearrange to appear as a quadratic equation $$x^2-x-1=0$$ Use the quadratic formula to find the values of $x$ $$x=\frac{-(-1)\pm\sqrt{(-1)^2-4·1·-1}}{2·1}$$ Watch those signs $$x=\frac{1\pm\sqrt{5}}{2}$$

Fibonacci Series & $φ$ Limit

Golden Ratio Powers

Algebraic Functions for the Fibonacci Series

5.2.4 The Plastic Ratio 🔧

$$\rho=\frac{a}{b}=\frac{b+c}{a}=\frac{b}{c}$$ $$a>b>c>0$$ The only real solution of $x^3=x+1$

$\approx1.32472$

(Need to insert cubic formula in 2.3)