Answer
$3\sqrt{5}$ units
Work Step by Step
To find the distance between the two points, we use the distance formula:
$d=\sqrt{(x_2-x_1)^2+(y_2-y_1)^2}$
Here we use the two points $(-1,3)$ and $(5,\ -9)$:
$d=\sqrt{(5--1))^{2}+(-9-3)^{2}}$
$=\sqrt{6^{2}+(-12)^{2}}$
$=\sqrt{36+144}$
$=\sqrt{180}$
$=\sqrt{36*5}$
$=6\sqrt{5}$
This gives us the distance and hence the diameter. To find the radius, we divide in two:
$radius=diameter/2$
$=\frac{1}{2}(6\sqrt{5})$
$=3\sqrt{5}$
