Question

Tính : S = 1.2 + 2.3 + 3.4 + ... + 38.39 + 39.40

Answer 1

\(S=1\cdot2+2\cdot3+3\cdot4+....+39\cdot40\)
\(\Rightarrow3S=1\cdot2\cdot3+2\cdot3\cdot\left(4-1\right)+....+39\cdot40\cdot\left(41-38\right)\)
\(=1\cdot2\cdot3+2\cdot3\cdot4-1\cdot2\cdot3+.....+39\cdot40\cdot41-38\cdot39\cdot40=39\cdot40\cdot41=63960\)

Answer 2

S=21320

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Answer 3

\(\Rightarrow S=\frac{63960}{3}=21320\)

Answer 4

S = 1.2 + 2.3 + 3.4 + ... + 38.39 + 39.40 

3S = 1.2.3 + 2.3.(4 - 1) + 3.4.(5 - 2) + ... + 38.39.(40 - 37) + 39.40.(41 - 38)

3S = 1.2.3 + 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

3S = 63960

S = 21320