Problem
19. Find the equation of a circle whose center is at the origin and radius is 4 . 19. A. $x^{2}+y^{2}=4$ B. $x^{2}+y^{2}=16$ C. $(x-4)^{2}+(y-4)^{2}=16$ D. $4 x+4 y=16$
Solution
Step 1 ：Given center (0,0) and radius 4, the equation of the circle is $(x-0)^{2}+(y-0)^{2}=4^{2}$
Step 2 ：Simplifying, we get \(x^{2}+y^{2}=16\) as the equation of the circle.
Step 3 ：\(\boxed{x^{2}+y^{2}=16}\)
