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Aflati ca numarul C=5xy(bara sus)+x3y(bara sus)+xy7(bara sus) setedivizibil cu 3, oricare ar fi cifrele x si y, x= 0.

Răspuns :

[tex]\overline{5xy}+\overline{x3y}+\overline{xy7}=\\ =500+10x+y+100x+30+y+100x+10y+7=\\ =210x+12y+537=\\ =3(70x+4y+149)=>(\overline{5xy}+\overline{x3y}+\overline{xy7})\vdots 3[/tex]