This returns an approximation of the derivative at a specific value. (89) :nDeriv(expression,variable) This returns an approximation of the derivative. d tends to return an exact derivative, so will likely be used instead. (84) :nDeriv(expression,variable,value,secantLength) This returns an approximation of the derivative at a specific value. Since the numerical derivative uses a secant to determine the approximation, this allows the secantLength to be set. (89) :nDeriv(expression,variable,secantLength)