En utilisant les congruences modulo 9 et 11 trouver le chiffre $x$.
  1. $(\overline{51840})_{10} \times (\overline{273581})_{10} = (\overline{1418243x040})_{10}$.

  2. $(\overline{2x99561})_{10} = [3(523 + x) ]^2$.

  3. $(2784x)_{10} = x\times (\overline{5569})_{10}.$

  4. $(\overline{512})\times (\overline{1x53125})_{10} = (\overline{1000000000})_{10}.$