# Thread: Help with spheres - calc 3

1. ## Help with spheres - calc 3

Find the equation of a sphere that contains the points (-3,0,0) (3,4,4) and (-4,3,4) if its center lies on the xz plane

This class has just started and i have no idea where to start with this becuase this is our first homework!! I am trying to find a point equidistant from all three points that could be the center, but I could only find a place which I found to be 2z+3x=4

Please help!! I am really frustrated =(

2. You probably need to know the distance formula in 3D, or the equation of a sphere, which is $(x-a)^2+(y-b)^2+(x-c)^2=r^2$.

The center on the xz plane says that the center (a, b, c)=(a, 0, c).

Plug in the three points now and solve for a, c, and r.

If you do not have the eqn of a sphere, you need to use the distance formula, setting up the distance between a point on the sphere (x, y, z), and the center (a, 0, c). Then substitution your three points for x, y, & z will give you three equations, with three unknowns, again a, c, and r.