GỌi dãy tính là A

3A = 6 x 7 x 3 + 7 x 8 x 3 + ... + 21x22x3

3A = 6x7x8 - 5x6x7 + 7x8x9-6x7x8 + ... + 21x22x23 - 20x21x22

3A = 21x22x23 - 5x6x7

3A = 10416

A = 3472

Gọi dãy tính là A

3A = 6 * 7 * 3 + 7 * 8 * 3 + ... + 21 * 22 * 3

3A = 6 * 7 * 8 - 5 *6 *7 + 7 * 8 * 9 - 6 * 7 * 8 + ... + 21 * 22 * 23 - 20 * 21 * 22

3A = 21 * 22 * 23 - 5 * 6 * 7

3A = 10461

A = 3472

GỌI DÃY TÍNH TRÊN LÀ A

3A=6*7*3+7*8*3+....+21*22*23

3A=6*7*8+5*6*7+7*8*9-6*7*8+.....21*22*23-20*21*22

3A=21*22*33-5*6*7

3A=10416

A=3472

1 x 2 = 2 

5 x 6 = 30 

2 x 3 = 6 

6 x 7 = 42 

3 x 4 = 12 

7 x 8 = 56 

4 x 5 = 20 

8 x 9 = 72 

HT

TL

1x2=2           2x3=6               3x4=12                     4x5=20

5x6=30             6x7=42                    7x8=56                      8x9=72

nha

HT

2                     6                         12                                20

30                    42                        56                                72 

k mình nha !!

=1/6-1/7+1/7-1/8+1/8-1/9+...+1/10-1/11

=1/6-1/11=5/66

   \(\dfrac{1}{5\times6}+\dfrac{1}{6\times7}+\dfrac{1}{7\times8}+\dfrac{1}{8\times9}+\dfrac{1}{9\times10}\)

= \(\dfrac{1}{5}-\dfrac{1}{6}\times\dfrac{1}{6}-\dfrac{1}{7}\times\dfrac{1}{7}-\dfrac{1}{8}\times\dfrac{1}{8}-\dfrac{1}{9}\times\dfrac{1}{9}-\dfrac{1}{10}\)

= \(\dfrac{1}{5}-\dfrac{1}{10}\)  

= \(\dfrac{1}{10}\)

Bài tập cuối tuần 10

`1/[4xx5]+1/[5xx6]+1/[6xx7]+1/[7xx8]+1/[8xx9]+1/[9xx10]`

`=1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10`

`=1/4-1/10`

`=10/40-4/40=6/40=3/20`

330 nha

Đặt  \(A=\frac{1}{2x3}+\frac{1}{3x4}+\frac{1}{4x5}+\frac{1}{5x6}+\frac{1}{6x7}+\frac{1}{7x8}+\frac{1}{8x9}\)

\(A=\frac{3-2}{2x3}+\frac{4-3}{3x4}+\frac{5-4}{4x5}+\frac{6-5}{5x6}+\frac{7-6}{6x7}+\frac{8-7}{7x8}+\frac{9-8}{8x9}\)

\(A=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}\)

\(A=\frac{1}{2}-\frac{1}{9}=\frac{9}{18}-\frac{2}{18}=\frac{7}{18}\)

1/2x3 + 1/3x4 + 1/4x5 +1/5x6 +1/6x7+1/7x8 + 1/8x9 = 7/18

giup minh voi mn oi