Question

Which of the following is the equation of a circle whose center is at the origin and whose radius is 4?

Answer 1

$\bf \textit{equation of a circle}\\\\ (x- h)^2+(y- k)^2= r^2 \qquad center~~(\stackrel{0}{ h},\stackrel{0}{ k})\qquad \qquad radius=\stackrel{4}{ r} \\\\\\ (x-0)^2+(y-0)^2=4^2\implies x^2+y^2=16$