Răspuns :
Fa suma lui gaus pt numerele de la 1 la 61 si apoi afli cate numere contine sirul stiind ca se adauga 3 la numarul urmator si scazi cele daoua sume!
3×0+1 + 3×1+1 + 3×2+1+....+ 3×20+1
3× ( 0+1+2+....+20) + 21
3× 20(20+1):2 +21
3× 20×21÷2 +21
3×10×21+21
30×21+21
630+21=651
3× ( 0+1+2+....+20) + 21
3× 20(20+1):2 +21
3× 20×21÷2 +21
3×10×21+21
30×21+21
630+21=651