1. x , y ∈ N
x - y = 340 si x = 4y + 1
4y + 1 - y = 340 ; 3y +1 = 340 ; 3y = 340 - 1 ; 3y = 339
y = 113
x = y + 340 = 113 + 340 = 453
2 . x - y = 1999 x = 5y + 7
5y + 7 - y = 1999 ; 4y + 7 = 1999 ; 4y = 1999 - 7
4y = 1992 ; y = 1992 : 4 = 498
x = 1999 + y = 1999 + 498 = 2497
3. x - y = 817 x = 55y + 7
55y + 7 - y = 817
54y = 817 - 7 ; 54 y = 810 ; y = 810 :54
y = 15
x = 817 + y = 817 + 15 = 832