En primer lugar, factorizamos los números
 
$39 = 3\cdot13$
$121 = 11^{2}$
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No hay factores comunes (el m.c.d. es 1)
$M.C.D. ( 39 , 121 ) = 1$

$\left. \begin{array}{c|c}39 & 3\cr13 & 13 \cr1\end{array} \right. \quad$$\left. \begin{array}{c|c}121 & 11\cr11 & 11 \cr1\end{array} \right. \quad$