For what value of A is the plane 2x-2y+z=3 is tangent to the sphere x^2+y^2+z^2 = A^2
问题描述：
For what value of A is the plane 2x-2y+z=3 is tangent to the sphere x^2+y^2+z^2 = A^2
答
The center of the sphere in (0,0,0),radius A.The distance from (0,0,0) to the plane is:
A = |2*0 - 2*0 + 0 - 3|/√[2² + (-2)² + 1²] = 1