[tex]\bf \textit{circumference of a circle}\\\\
C=2\pi r\qquad
\begin{cases}
r=radius\\
-----\\
r=5x^2
\end{cases}\implies C=2\pi 5x^2\implies C=10\pi x^2
\\\\\\
\textit{area of a circle}\\\\
A=\pi r^2\qquad
\begin{cases}
r=radius\\
-----\\
r=5x^2
\end{cases}\implies A=\pi (5x^2)^2\implies A=\pi (5^2x^{2\cdot 2})
\\\\\\
A=\pi (25x^4)\implies A=25\pi x^4 [/tex]
