Question

Find an equation of the sphere with center (4, −12, 8) and radius 10. use an equation to describe its intersection with each of the coordinate planes. (if the sphere does not intersect with the plane, enter dne.) intersection with xy-plane intersection with xz-plane intersection with yz-plane

Answer 1

Sphere: (x - 4)^2 + (y + 12)^2 + (z - 8)^2 = 100 Intersection in xy-plane: (x - 4)^2 + (y + 12)^2 = 36 Intersection in xz-plane: DNE Intersection in yz-plane: (y + 12)^2 + (z - 8)^2 = 84 The desired equation is quite simple. Let's first create an equation for the sphere centered at the origin: x^2 + y^2 + z^2 = 10^2 Now let's translate that sphere to the desired center (4, -12, 8). To do that, just subtract the center coordinate from the x, y, and z variables. So (x - 4)^2 + (y - -12)^2 + (z - 8)^2 = 10^2 (x - 4)^2 + (y - -12)^2 + (z - 8)^2 = 100 Might as well deal with that double negative for y, so (x - 4)^2 + (y + 12)^2 + (z - 8)^2 = 100 And we have the desired equation. Now for dealing with the coordinate planes. Basically, for each coordinate plane, simply set the coordinate value to 0 for the axis that's not in the desired plane. So for the xy-plane, set the z value to 0 and simplify. So let's do that for each plane: xy-plane: (x - 4)^2 + (y + 12)^2 + (z - 8)^2 = 100 (x - 4)^2 + (y + 12)^2 + (0 - 8)^2 = 100 (x - 4)^2 + (y + 12)^2 + (-8)^2 = 100 (x - 4)^2 + (y + 12)^2 + 64 = 100 (x - 4)^2 + (y + 12)^2 = 36 xz-plane: (x - 4)^2 + (y + 12)^2 + (z - 8)^2 = 100 (x - 4)^2 + (0 + 12)^2 + (z - 8)^2 = 100 (x - 4)^2 + 12^2 + (z - 8)^2 = 100 (x - 4)^2 + 144 + (z - 8)^2 = 100 (x - 4)^2 + (z - 8)^2 = -44 And since there's no possible way to ever get a sum of 2 squares to be equal to a negative number, the answer to this intersection is DNE. This shouldn't be a surprise since the center point is 12 units from this plane and the sphere has a radius of only 10 units. yz-plane: (x - 4)^2 + (y + 12)^2 + (z - 8)^2 = 100 (0 - 4)^2 + (y + 12)^2 + (z - 8)^2 = 100 (-4)^2 + (y + 12)^2 + (z - 8)^2 = 100 16 + (y + 12)^2 + (z - 8)^2 = 100 (y + 12)^2 + (z - 8)^2 = 84