1. a - b = 5; a + b = 2005 => 2a = 2010( am adunat una sub cealalta cele 2 relatii matematice) => a = 1005 => b = 1000;
2. ab+ac+ad=2005 <=> a( b + c + d ) = 2005; folosim b+c+d=401; => a = 2005 : 401 =>
a = 5;
3. abc+bca+cab = 111( a + b + c) = 111* 12 = 1332;( am folosit dezvoltarea in baza 10);
Bafta!