Corrigé du 23 P. 44
retour
\[\begin{aligned} u_3 &= u_4 - r = 25 - 6 = 19\;;& \\ u_2 &= u_3 - r = 19 - 6 = 13\;;& \\ u_1 &= u_2 - r = 13 - 6 = 7.& \end{aligned}\]