# Need to get an equation for when it is tangent

1. Feb 5, 2008

### bharp24

I need someone to help me!! i have been given a center point of (3,-1,-2) of a sphere and i need to get an equation for when it is tangent to the a) the yz plane b) y=7. i know i need to use the distance formuala to attain the radius, but i am majorly confusing myself in what coordinate to pick. Do i just use (0,0,0) as tangent point to the plane for a)?? please help!

2. Feb 5, 2008

### rocomath

What is the general formula for a sphere?

3. Feb 5, 2008

### bharp24

what i have so far is (x-3)^2 + (x+1)^2 + (x+2)^2= r^2. i just need to find the radius of this equation, so that the sphere will be tangent to the yz plane. once i know the coordinates to use for the tangent point, i can use the distance formula to give me the radius.

4. Feb 5, 2008

### rocomath

It would actually be ...

$$(x-3)^2+(y+1)^2+(z+2)^2=r^2$$

If you want it tangent to the yz-plane, what variable must be 0?

5. Feb 5, 2008

### bharp24

that is what i meant..sorry. i still need to know what the equation would be though to figure out when it is tangent to the yz plane.

6. Feb 6, 2008

### HallsofIvy

Staff Emeritus
So you want a single point to be on the yz plane. What is the equation of the yz plane?
At what point of the sphere is that equation true? What is the distance between that point and the center of the sphere?