Question

tính S = 1.2+2.3+3.4+...+39.40

Answer 1

Ta có: S = 1.2+2.3+3.4+...+39.40

=> 3S = 1.2.(3 - 0) + 2.3.(4 - 1) + ..... + 39.40.(41 - 38)

=> 3S = 1.2.3 - 1.2.3 + 2.3.4 - 2.3.4 + ..... + 39.40.41

=> 3S = 39.40.41

=> S = 39.40.41/3

=> S = 21320

Answer 2

\(S=1.2+2.3+...+39.40\)

\(3S=1.2.\left(3-0\right)+2.3\left(4-1\right)+...+39.40\left(41-38\right)\)

\(3S=1.2.3-0.1.2+2.3.4-1.2.3+...+39.40.41-38.39.40\)

\(S=\frac{39.40.41}{3}\)

\(\Rightarrow S=21320\)

Answer 3

Ta có: S = 1.2+2.3+3.4+...+39.40

         S = 1.2+2.3+3.4+...+39.40

         S = 1.2+2.3+3.4+...+39.40

=> Ta có 3S là: 

3S = 1.2.(3 - 0) + 2.3.(4 - 1) + ..... + 39.40.(41 - 38)

 3S = 1.2.3 - 1.2.3 + 2.3.4 - 2.3.4 + ..... + 39.40.41

 3S = 39.40.41

 S = 39.40.41:3

 S = 21320

Answer 4

Ta có :

\(S=1.2+2.3+...+39.40\)

\(\Rightarrow3S=1.2\left(3-0\right)+2.3\left(4-1\right)+...+39.40\left(41-38\right)\)

\(\Rightarrow3S=1.2.3-0.1.2+2.3.4-1.2.3+...+39.40.41-38.39.40\)

\(\Rightarrow S=\frac{39.40.41}{3}\)

\(\Rightarrow S=21320\)