 If the curve is given parametrically by
then the slope of the tangent is
giving the equation for the tangent line at formula_26 as
If formula_28 the tangent line is not defined.However, it may occur that the tangent line exists and may be computed from an implicit equation of the curve.The line perpendicular to the tangent line to a curve at the point of tangency is called the "normal line" to the curve at that point.