Posted by **sphere** on Tuesday, August 23, 2011 at 11:56am.

First of all let me apologize if this is the wrong section; this is a review question from a Calculus III class, but it doesn't involve any actual calculus so I posted it here.

If I have a sphere with center (2,-10,4) and radius 10 what is:

the intersection with the yz plane?

the intersection with the xy plane?

I get tripped up with geometry in more than 2 dimensions so I'm confused as how to handle this.

Should I treat the "omitted plane" (my term for lack of a better word) value as zero? If so does that change the radius? I'm guessing not given that the distance formula for a circle or a sphere is equal to r^2.

So for yz would I say

(y+10)^2 + (z-4)^2 = 100

And if so, how do I find the yz plane intersection from that? I know I've done this before but I don't recall how to do it.

- geometry -
**Reiny**, Tuesday, August 23, 2011 at 1:23pm
the equation of the sphere is

(x-2)^2 + (y+10)^2 + (z-4)^2 = 100

so in the xz plane: y = 0

(x-2)^2 + 100 + (z-4)^2 = 100

(x-2)^2 + (x-4)^2 = 0

conclusion: the yz plane just touches the sphere.

for xy plane , let z=0

(x-2)^2 + (y+10)^2 + 16 = 100

(x-2)^2 + (y+10)^2 = 84

Visualize the sphere having a slice cut off, and you are looking at the surface of the cut.

Wouldn't that be a circle?

And unless the cut is made through the middle of the sphere, wouldn't that circle have a radius less than 10 ?

So for my answer of (x-2)^2 + (y+10)^2 = 84, the centre of that surface would be (2, -10,0) and the radius of the cut surface would be √84

you had the right idea, but you just dropped the whole bracketted term, you should have just let the y=0

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