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Determinna numerele naturale x si y , stiind ca xy+3x+3y+9=21

Răspuns :

10x+y+3x+3y+9=21
13x+4y+9=21
13x+4y=21-9
13x+4y=12=>x=0, 13•k>12,k>0 => 4y=12
y=12:4
y=3