The problem of the tangent
 
For us, Leibniz was a great mathematician. 
Because of his results in the  infinitesimal calculus, our math class started to study on Leibniz` theories this year and we decided to work with those theories in detail. Leibniz solved the problem of the tangent at the same time as Newton, a problem which at that time had been solved for special graphs only. 

Leibniz` thoughts were the following: 

 

The subnormal QR, the normal PR and also the ordinate PQ form a rectangular triangle. This triangle is similar to all other rectangular triangles which consist of the tangent  and the parts on the parallels to the ordinate and to the abscissa. The relation of the "Katheten" of both triangles is the same and remains identical even when dx gets very small in the smaller triangle.  The closer dx approaches 0, the closer the gradient of ds approaches the gradient of the curve in P. 
Hereby Leibniz proved that one can find the gradient of a special point within every continuous curve. This was the beginning of the infinitesimal calculus. 

   (Claudia Ruch, Elisabeth Schallhart, Jennifer Galambos)