Chapter 14 - Sequences, Series, and the Binomial Theorem - 14.2 Arithmetic Sequences and Series - 14.2 Exercise Set - Page 903: 55

$x^2+y^2=16$

The standard equation of the circle has the form $(x-x_0)^2+(y-y_0)^2=R^2,$ where $(x_0,y_0)$ is centre coordinate and $R$ - radius. In our case $x_0=0, y_0=0$ and $R=4$. Hence the circle's equation is: $$x^2+y^2=16$$