Fie S=2^1+2^2+2^3+2^4...+2^120
Grupez termenii câte 3, dând factor p3 fel mai mic
S=(2^1+2^2+2^3+2^4)+...+(2^117+2^118+2^119+2^120)
S=2^1(1+2+2^2+2^3)+...+2^117(1+2+2^2+2^3)
S=(1+2+2^2+2^3)(2^1+2^5+...+2^117)
S=(1+2+4+8)(2^1+2^5+...+2^117)
S=15*(2^1+2^5+...+2^117) |15