1 + 3 + 5 +... + 15 +17 +19=S
19+ 17 + 15 + ... + 3 + 2 + 1 =S
------------------------------------------ +
20+20+20+...+ 20+20+20=2S (20 apare de (19+1):2=10 ori)
20*10=2S
200=2S => S=200:2=100
1 + 3 + 5 + ... + 19 =
[ ( 19 - 1 ) : 2 + 1 ] × [ ( 19 + 1 ) : 2 ] =
( 18 : 2 + 1 ) × ( 20 : 2 ) =
( 9 + 1 ) × 10 =
10 × 10 = 100