 In the 1630s Fermat developed the technique of adequality to calculate tangents and other problems in analysis and used this to calculate tangents to the parabola.The technique of adeqality is similar to taking the difference between formula_1 and formula_2 and dividing by a power of formula_3.Independently Descartes used his method of normals based on the observation that the radius of a circle is always normal to the circle itself.