Question

S = 1*2+2*3+3*4+...+39*40

Answer 1

Ta có: S = 1.2 + 2.3 + 3.4 +.....+ 39.40

=> 3S = 3.(1.2 + 2.3 + 3.4 +.....+ 39.40)

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

=> 3S = 0.1.2 + 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

Ta có: S = 1.2 + 2.3 + 3.4 +.....+ 39.40

=> 3S = 3.(1.2 + 2.3 + 3.4 +.....+ 39.40)

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

=> 3S = 0.1.2 + 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 3

Ta có: S = 1.2 + 2.3 + 3.4 +.....+ 39.40

=> 3S = 3.(1.2 + 2.3 + 3.4 +.....+ 39.40)

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

=> 3S = 0.1.2 + 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

21320

Answer 5

S = 1*2+2*3+3*4+...+39*40

Ta có: S = 1.2 + 2.3 + 3.4 +.....+ 39.40

=> 3S = 3.(1.2 + 2.3 + 3.4 +.....+ 39.40)

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

=> 3S = 0.1.2 + 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