Răspuns :
[tex]A=27+2(1+2+3+...+26)=27+\frac{2*26*27}{2}=27+26*27=27*27=27^2=729[/tex]
1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20+21+22+23+24+25+26=351
27+2*351=729
27+2*351=729