This is a paper about the first attempts of the demonstration of the fundamental theorem of algebra. Before, we analyze the tie between complex numbers and the number of roots of an equation of -n-th degree. In second paragraph we see the relation between the integration and fundamental theorern. Finally, we observe the linear differential equation with constant coefficients and the Euler's position about the fundamental theorem and then we consider the d'Alembert's, Euler's and Laplace's demonstrations. lt is a synthesis paper dedicated to Pere Menal a collegue and a friend.