3sqrt(u[n]) + 4 > u[n]
3sqrt(u[n]) > u[n] - 4
sqrt(u[n]) > (1/3)u[n] - (4/3)
u[n] > [(1/3)u[n]]² - 2 * (1/3)u[n] * (4/3) + (4/3)²
u[n] > (1/9)u[n]² - (8/9)u[n] + 16/9
0 > (1/9)u[n]² - (17/9)u[n] + 16/9
Etudie (1/9)x² - (17/9)x + 16/9
et vois que c'est absurde 