- PopeLv 75 months ago
The meaning of your parenthetical (R) is not at all clear. Is that meant to be part of the equation in some way?
As for x = k, if k is a real number, then the equation corresponds to a vertical line in the x-y plane. The line has no gradient in the usual sense. To be clear about this, that does not mean zero gradient, and it does not mean the gradient is infinity. There is no gradient at all.