Question

determinati numarul xy din relatia xy=3x+5y

Answer 1

xy-3x-5y+15=15
x(y-3)-5(y-3)=15
(x-5)(y-3)=15
Avem urmatoarele posibilitati in numere naturale 15=1x15=3x5=5x3=15x1 adica $(x,y)\in \{(6,18),(8,8),(10,6),(20,4)\} \\ xy\in \{108,64,60,80\}$