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Efectuati:(3+6+9+...+30)-(2+4+6+...+20)

Răspuns :

1+2+3+,,,+n=n(n+1)/2
(3+6+9+...+30)-(2+4+6+...+20)=3(1+2+3+....+10)-2(1+2+3+...+10)=(1+2+3+...+10)(3-2)=1+2+3+...+10=10*11:2=55