The circle below is centered at the origin and has a radius of 5. What is its equation?

[tex]\bf \textit{equation of a circle}\\\\ (x- h)^2+(y- k)^2= r^2 \qquad center~~(\stackrel{0}{ h},\stackrel{0}{ k})\qquad \qquad radius=\stackrel{5}{ r} \\\\\\ (x-0)^2+(y-0)^2=5^2\implies x^2+y^2=25[/tex]