Question

A(-3, 9) and B(5, 9) are the endpoints of the diameter of a circle. Use these points to find the standard equation of the circle.

Answer 1

Check the picture below.

notice the midpoint of the diameter segment, is just 1,9.

also, recall the radius is half the diameter, notice how long the radius is, just 4 units.

$\bf \textit{equation of a circle}\\\\ (x-{{ h}})^2+(y-{{ k}})^2={{ r}}^2 \qquad \begin{array}{lllll} center\ (&{{ h}},&{{ k}})\qquad radius=&{{ r}}\\ &1&9&4 \end{array}$