[tex]\bf \cfrac{small~radius}{large~radius}\qquad 2:5\qquad \cfrac{2}{5}\qquad \cfrac{5}{x}=\cfrac{2}{5}\implies \cfrac{5\cdot 5}{2}=x\implies \cfrac{25}{2}=x\\\\
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\textit{circumference of a circle}\\\\
C=2\pi r\quad
\begin{cases}
r=radius\\
-----\\
r=\frac{25}{2}
\end{cases}\implies C=\underline{2}\pi \cdot \cfrac{25}{\underline{2}}\implies C=25\pi [/tex]
