0 < a < 180/2, is just another way of saying 0° < a < 90°, which is another way of saying angle "a" is in the I Quadrant, where cosine and sine or x,y are both positive.
[tex]\bf cos(a)=\cfrac{\stackrel{adjacent}{5}}{\stackrel{hypotensue}{13}}\impliedby \textit{now let's find the \underline{opposite side}}
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\textit{using the pythagorean theorem}
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c^2=a^2+b^2\implies \pm \sqrt{c^2-a^2}=b
\qquad
\begin{cases}
c=hypotenuse\\
a=adjacent\\
b=opposite\\
\end{cases}
\\\\\\
\pm\sqrt{13^2-5^2}=b\implies \pm\sqrt{144}=b\implies \pm 12=b\implies \stackrel{I~quadrant}{+12=b}
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sin(a)=\cfrac{\stackrel{opposite}{12}}{\stackrel{hypotenuse}{13}}[/tex]
