Table of Contents

Trigonometric Functions in Right-Angle Triangle


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  \coordinate[label={[black]below:$B$}] (B) at (-1,-1);
  \coordinate[label={[black]right:$C$}] (C) at (1,-1);
  
  % triangle 
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  %\drawbrace{A}{B}{2mm}{green}{$c$}{-4mm}{0}{mirror}
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\end{tikzpicture}

\begin{eqnarray*}
\sin{{\color{cyan}\alpha}} &=& \frac{\color{blue}{a}}{\color{red}{b}} = \frac{\overline{BC}}{\overline{AC}} & \\
\cos{{\color{cyan}\alpha}} &=& \frac{\color{green}{c}}{\color{red}{b}} = \frac{\overline{AB}}{\overline{AC}} & \\
\tan{{\color{cyan}\alpha}} &=& \frac{\color{blue}{a}}{\color{green}{c}} = \frac{\overline{BC}}{\overline{AB}}
\end{eqnarray*}

\begin{eqnarray*}
\csc{{\color{cyan}\alpha}} &=& \frac{\color{red}{b}}{\color{blue}{a}} = \frac{\overline{AC}}{\overline{BC}} & \\
\sec{{\color{cyan}\alpha}} &=& \frac{\color{red}{b}}{\color{green}{c}} = \frac{\overline{AC}}{\overline{AB}} & \\
\cot{{\color{cyan}\alpha}} &=& \frac{\color{green}{c}}{\color{blue}{a}} = \frac{\overline{AB}}{\overline{BC}} 
\end{eqnarray*}

\begin{eqnarray*}
\tan{{\color{cyan}\alpha}} &=& \frac{\sin{{\color{cyan}\alpha}}}{\cos{{\color{cyan}\alpha}}}
\end{eqnarray*}

Pythagoras (Law of Cosines)



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  %  (0.1,-0.1) -- (-0.1,0.1);
  %\coordinate[label={[red]above:$O$}] (O) at (0,0);
  
  % coordinates
  \coordinate[label={[black]left:$A$}] (A) at (-1,1);
  \coordinate[label={[black]below:$B$}] (B) at (-1,-1);
  \coordinate[label={[black]right:$C$}] (C) at (1,0);
  
  % triangle 
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\end{tikzpicture}

\begin{eqnarray*}
{\color{blue}a}^{2} &=& {\color{green}c}^{2} + {\color{red}b}^{2} - 2{\color{green}c}{\color{red}b}\cos{{\color{cyan}\alpha}}
\end{eqnarray*}

in a right-angle triangle, $\alpha=90^{\circ}$ such that $\cos{{\color{cyan}\alpha}}=0$:

\begin{eqnarray*}
{\color{blue}a}^{2} &=& {\color{green}c}^{2} + {\color{red}b}^{2}
\end{eqnarray*}

Medians in a Triangle


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  \coordinate[label={[black]right:$C$}] (C) at (3,0);
  \coordinate[label={[black]above:$M_{a}$}] (Ma) at ($ (B) !.5! (C) $);
  \coordinate[label={[black]below left:$M_{b}$}] (Mb) at ($ (A) !.5! (C) $);
  \coordinate[label={[black]below right:$M_{c}$}] (Mc) at ($ (A) !.5! (B) $);
  
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  \draw[black, line width=.1mm] (A) -- (B) -- (C) -- cycle;
  
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  \drawpoint{Mc}{.5mm}{black}

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  \drawbrace{Ma}{C}{3mm}{blue}{$a''$}{0}{-5mm}{mirror}
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  % braces for segment medians b
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  \drawbrace{C}{Mc}{1mm}{black}{$m_{c}$}{0}{-3mm}{}
  
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\begin{eqnarray*}
m_{a} &=& \sqrt{\frac{2{\color{red}b}^{2}+2{\color{green}c}^{2}-{\color{blue}a}^{2}}{4}} 
\end{eqnarray*}

\begin{eqnarray*}
a' &\equiv& a'' & \\
b' &\equiv& b'' & \\
c' &\equiv& c''
\end{eqnarray*}

\begin{eqnarray*}
\frac{3}{4}({\color{blue}a}^{2} + {\color{red}b}^{2} + {\color{green}c}^{2}) &=& m_{{\color{blue}a}}^{2} + m_{{\color{red}b}}^{2} + m_{{\color{green}c}}^{2}
\end{eqnarray*}

\begin{itemize}
  \item $G$ represents the centre of mass also called the centroid. The entire weight of the triangle can be balanced in $G$ by pinning a pivot in $G$.
  \item The medians $m_{a}$, $m_{b}$ and $m_{c}$ divide the triangle $\widehat{ABC}$ into $6$ smaller triangles of equal surface.
\end{itemize}

Area of a Triangle



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\begin{eqnarray*}
A_{\widehat{ABC}} &=& \frac{{\color{red}h}{\color{blue}a}}{2}
\end{eqnarray*}

Using Heron's formula:

\begin{eqnarray*}
A_{\widehat{ABC}} &=& \sqrt{p(p-{\color{blue}a})(p-{\color{red}b})(p-{\color{green}c})} \text{ and } p=\frac{{\color{blue}{a}}+{\color{red}{b}}+{\color{green}{c}}}{2}
\end{eqnarray*}

A complete derivation of the Heron formula can be found in the mathematics section on triangles.

Theorem of Ceva


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  \coordinate[label={[black]above:$D$}] (D) at ($ (B) !.52! (C) $);
  \coordinate[label={[black]below left:$E$}] (E) at ($ (A) !.4! (C) $);
  \coordinate[label={[black]below right:$F$}] (F) at ($ (A) !.4! (B) $);
  
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  \coordinate[label={[black]above:$O$}] (O) at (O);
  
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\begin{eqnarray*}
\frac{{\color{green}c}}{{\color{green}c'}} * \frac{{\color{blue}a}}{{\color{blue}a'}} * \frac{{\color{red}b}}{{\color{red}b'}} &=& 1
\end{eqnarray*}

The point $O$ represents the intersection of three arbitrarily drawn lines from the vertices ($A$, $B$ or $C$) of the triangle \widehat{ABC}.

Theorem of Menelaus



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  \drawbrace{C}{R}{2mm}{green}{$r$}{0}{-4mm}{mirror}
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\begin{eqnarray*}
\frac{{\color{blue}a'}}{{\color{blue}a}}*\frac{{\color{orange}s}}{{\color{violet}}t}}*\frac{{\color{teal}u}}{{\color{olive}v}} &=& 1
\end{eqnarray*}

Theorem of Parallels



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If $\overline{PQ}$ is parallel to $\overline{BC}$ then:

\begin{eqnarray*}
\frac{\overline{AP}}{\overline{AB}}=\frac{\overline{AQ}}{\overline{AC}}&=&\frac{\overline{PQ}}{\overline{BC}}
\end{eqnarray*}