Réponses

2014-09-03T14:16:38+02:00
On suppose x>y.
On note z=x-y
donc x=z+y
IxI=Iy+zI
On sait que Iy+zI≤IyI+IzI
Donc IxI≤IyI+IzI
⇔IxI-IyI≤IzI
⇔IxI-IyI≤Ix-yI