1*2 + 2*3 +3*4+...+38*39+39*40
S=1×2+2×3+3×4+...+38×39+39×40
S = 1*2+2*3+3*4+.....+38*39+39*40
Tính S = 1*2+2*3+3*4+4*5+...+38*39+39*40
Tính: S=1*2+2*3+3*4+.........+38*39+39*40
tình S=1*2+2*3+3*4+......+38*39+39*40
tính S=1*2+2*3+3*4+.....+38*39+39*40
Tính : S = 1*2+2*3+3*4+...+38*39+39*40
S= 1*2+2*3+3*4+...+38*39+39*40
S= ?