Ballantine & Jerbert 1952 do not mention this restriction in their article Ballantine, J.P.; Jerbert, A.R. (1952), “Distance from a line or plane to a point”, 《American Mathematical Monthly》 59: 242–243, doi:10.2307/2306514
Ballantine & Jerbert 1952 Ballantine, J.P.; Jerbert, A.R. (1952), “Distance from a line or plane to a point”, 《American Mathematical Monthly》 59: 242–243, doi:10.2307/2306514