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2013-11-23T16:39:46+01:00
A = 13/7-(4/3+11/11)
   =13/7-(4/3+1)
   =13/7-(4/3+3/3)
   =13/7-7/3
   =39/21-49/21=-10/21
B = (-5/4)x(-4/7)x(+11/15)
   =20/28x11/15
   =220/420 ou 11/21