A variable plane at unit distance from the origin cuts the co-ordinate axes at A, B and C. The…

A variable plane at unit distance from the origin cuts the co-ordinate axes at A, B and C. The centroid of the triangle formed by joining the points A, B and C satisfies the relation $\frac{1}{x^2} + \frac{1}{y^2} + \frac{1}{z^2} = k$, then what is the value of $k$?