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The Folium of Descartes
The folium of Descartes is a famous curve named after the French philosopher and mathematician Rene Descartes (pictured top right). As well as significant contributions to philosophy (“I think therefore I am”) he was also the father of modern geometry through the development of the x,y coordinate system of plotting algebraic curves. As such the Cartesian plane (as we call the x,y coordinate system) is named after him.
Pascal and Descartes
Descartes was studying what is now known as the folium of Descartes (folium coming from the Latin for leaf) in the first half of the 1600s. Prior to the invention of calculus, the ability to calculate the gradient at a given point was a real challenge. He placed a wager with Pierre de Fermat, a contemporary French mathematician (of Fermat’s Last Theorem fame) that Fermat would be unable to find the gradient of the curve – a challenge that Fermat took up and succeeded with.
Calculus – implicit differentiation:
Today, armed with calculus and the method of implicit differentiation, finding the gradient at a point for the folium of Descartes is more straightforward. The original Cartesian equation is:
which can be differentiated implicitly to give:
Therefore if we take (say) a =1 and the coordinate (1.5, 1.5) then we will have a gradient of -1.
Parametric equations
It’s sometimes easier to express a curve in a different way to the usual Cartesian equation. Two alternatives are polar coordinates and parametric coordinates. The parametric equations for the folium are given by:
In order to use parametric equations we simply choose a value of t (say t =1) and put this into both equations in order to arrive at a coordinate pair in the x,y plane. If we choose t = 1 and have set a = 1 as well then this gives:
x(1) = 3/2
y(1) = 3/2
therefore the point (1.5, 1.5) is on the curve.
You can read a lot more about famous curves and explore the maths behind them with the excellent “50 famous curves” from Bloomsburg University.
Essential Resources for IB Teachers
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Essential Resources for both IB teachers and IB students
1) Exploration Guides and Paper 3 Resources
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IB Maths Resources from Intermathematics 