Numerele care nu sunt divizibile nici cu 2 si nici cu 3 sunt:
1,5,7,11,13,17,19,23...
Suma e:
S = 1+7+13+19+25+...+(6n-5) + 5+11+17+23+29+...+(6n-1)
n = 50
S = 6(1+2+3+4+5+...+n) - 5-5-5-...-5 + 6(1+2+3+4+5+...+n) - 1-1-1-...-1
S = 12(1+2+3+4+5+...+n) - 5×50 - 1×50
S = 12×n(n+1)/2 - 6×50
S = 6n(n+1) - 6×50
S = 6(n(n+1) - 50)
S = 6×(50(50+1)-50)
S = 6×(50×51-50)
S = 6×50×50
S = 6×2500
S = 15000
