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Calculati suma:1+11+21+...+111

Răspuns :

1+11+21+…111=1+10+1+20+1+30+1+…+110+1=
=1*12+10+20+30+…+110=12+10(1+2+3…11)=
=12+10[(11*12)/2]=12+10*66=12+660=672