# Homework Help: What the formula for the distance from a point to a sphere

1. Aug 11, 2009

### khdani

i know this formula from a point to a plane
$$d=\frac{Ax_0+By_0+cz_0}{\sqrt{A^2+B^2+C^2}}$$

and i know that i could take from its center.

but i am looking for a formula of the distance from a point to a sphere
??

2. Aug 11, 2009

### Staff: Mentor

I'm not aware that there is such a formula, at least not one that is widely circulated. Assuming the point is outside the sphere, find the distance from the point to the center of the sphere, and then subtract the radius of the sphere.

3. Aug 11, 2009

### HallsofIvy

If the point is inside the sphere, do the subtraction the other way around. Or, like you do with the distance between two points on a number line, do the subtraction either way and take the absolute value.