S1=3+3+3+.....3 (de 25+1 ori)+4×(1+2+3+4+...+25)= =26×3+4×(25×26):2
=78+2×25×26=78+1300=1378
S = S1 - S2 = 1378 - S2
S2=(1+5+9+...+101)
termenii sunt din 4 in 4 S2=(4×0+1)+(4×1+1)+.....(4×25+1)= S2=4×0+4×1+4×2+......+4×25+1+1+1+1......+1(de 26 de ori) S2=4(0+1+2+3+......+25)+26×1 S2=4×[(25×26):2]+26 S2=1326