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2014-04-05T21:19:40-03:00
(31-x) / (4-x) = (139-x)/(31-x) = 
(31-x) = (4-x) . (139-x) / (31-x) = 
(31-x) = 556 - 4x - 139x + x² / (31-x) = 
961 - 62x + x² = 556 - 4x - 139x + x² 
961 - 556 = 62x -4x - 139x 
405 = 58x - 139x 
405 = -81x 
x = -5 
1 5 1