Ta có
\(S=1.2+2.3+....+39.40\)
\(\Rightarrow3S=1.2\left(3-0\right)+2.3\left(4-1\right)+....+39.40\left(41-38\right)\)
\(\Rightarrow3S=1.2.3-0.1.2+2.3.4-1.2.3+....+39.40.41-38.39.40\)
\(\Rightarrow S=\frac{39.40.41}{3}\)
=> S-21320
Vaayj S=21320
Ta có
\(S=1.2+2.3+3.4+...+39.40\)
\(\Rightarrow3S=1.2.3+2.3.3+3.4.3+...+39.40.3\)
\(\Rightarrow3S=1.2.\left(3-0\right)+2.3.\left(4-2\right)+3.4.\left(5-2\right)+...+39.40.\left(41-38\right)\)
\(\Rightarrow3S=1.2.3-0.1.2+2.3.4-1.2.3+......+39.40.41-38.39.40\)
\(\Rightarrow3S=38.39.40\)
\(\Rightarrow S=\frac{38.39.40}{3}\)
\(\Rightarrow S=19760\)
Vậy S=19760
3S = 1.2.(3-0) + 2.3.(4-1) + 3.4.(5-2) + ... + 38.39.(40-37) + 39.40.(41-38)
3S = 1.2.3 - 0.1.2 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ... + 38.39.40 - 37.38.39 + 39.40.41 - 38.39.40
3S = 39.40.41
S = 13.40.41 = 20800
