Răspuns :
1+3+5+7+...+19 = (1+19) + (3+17) +(5+15)+ (7+13)+ (9+11) = 20+20+20+20+20 = 20 x 5 = 100 =10² = k²
Progresie aritmetica cu ratia 2.
S=((19-1):2 + 1)(1+19)/2 = 5x20=10^2
S=((19-1):2 + 1)(1+19)/2 = 5x20=10^2