Current divider rule calculation issue
Sorry... I just figured out what was wrong... It's not : 

\$I_{R1} = \dfrac{\dfrac{1}{\cfrac{1}{5000}+\cfrac{1}{10000}}}{\dfrac{1}{5000}} \times 3mA\$

But : 

\$I_{R1} = \dfrac{\dfrac{1}{\cfrac{1}{5000}+\cfrac{1}{10000}}}{5000} \times 3mA\$

And in that case I've got the correct answer.

Sorry.