Chapter 12.3, Problem 54E

If $r=\langle x,y,z\rangle ,a=\langle a_1,a_2,a_3\rangle ,$ and $b=\langle b_1,b_2,b_3\rangle ,$ show that the vector equation $\left(r-a\right)\cdot \left(r-b\right)=0$ represents a sphere, and find its center and radius.