u_1 = 0.9 (slider from -10 to 20, step 0.1)
r = 2.4 (slider from -10 to 10, step 0.1)
u_{n+1}(x) = u_1 + (x - 1.0) * r
n = 3 (slider from 0 to 20, step 1)
u_n: (n, u_{n+1}(n))
texte1: "u_{n+1} - u_n = " + r