Question

calculati suma 1+3+5+7+9+...21

Answer 1

1+3+5+...+(2n-1)=n^2, unde n>0 si nr.nat.
2n-1=21
2n=22
n=11

1+3+5+...+21=11^2=121

Answer 2

1 +3 + 5 + 7 + 9 + ....+ 21
n = ( 21 - 1 ) : ( 3 -1 ) +1
n = 20 : 2 +1
n = 10 +1
n = 11
S = [( 21 +1 ) x 11] : 2
S = (22 x 11) : 2
S = 242 : 2
S = 121.