Question

Tính :

S= 1x2+2x3+3x4+...+38x39+39x40

Answer 1

S= 1.2 + 2.3 + ... + 39.40

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

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

=> 3S = 1.2.3 - 0.1.2 + 2.3.4 - 1.2.3 + ... + 39.40.41 - 38.39.40

=> 3S = 39.40.41

=> S = \(\frac{39.40.41}{3}\)

=> S=21320

Answer 2

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

=>3S=1.2.3+2.3.3+3.3.4+...+3.38.39+39.40.3

=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

=39.40.41

=>S=13.40.41

Answer 3

S = 1 x 2 + 2 x 3 = 3 x 4 + ... + 38 x 39 + 39 x 40

3S = 1 x 2 x 3 + 2 x 3 x 3 + 3 x 4 x 3+ ... + 38 x 39 x 3 + 39 x 40 x 3

3S = 1 x 2 x 3 + 2 x 3 x ( 4 - 1 ) + 3 x 4 x ( 5 - 2 ) + ... + 38 x 39 x ( 40 - 37 ) + 39 x 40 x ( 41 - 38 )

3S = 1 x 2 x 3 + 2 x 3 x 4 - 1 x 2 x 3 + 3 x 4 x 5 - 2 x 3 x 4 + ... + 38 x 39 x 40 - 37 x 38 x 39 + 39 x 40 x 41 - 38 x 39 x 40

 S = 39 x 40 x 41 : 3 

 S = 21320

Answer 4

Đúng 100 % sai ko chịu trách nhiệm
 

Answer 5

S = 1 x 2 + 2 x 3 + 3 x 4 + ... + 38 x 39 x 40

S = 2 + 6 + 12 + ... + 59280

S = ( 2 + 12 ) + 6 + .... + 59280

S = 14 + 6 + 20 + 59280

S = ( 14 + 6 ) + ( 20 + 59280 )

S = 20 + 59300

S = 59320

Answer 6

Có bị cái gì ko hả Mai Thị Thanh Hoa tự nhiên lấy 38 x 39 x 40 = 59280 :v