1/2×3 + 2/3×5 + 3/5×8 + 4/8×12 + ... + 10/47×57

= 1/2 - 1/3 + 1/3 - 1/5 + 1/5 - 1/8 + 1/8 - 1/12 + ... + 1/47 - 1/57

= 1/2 - 1/57

= 57/114 - 2/114

= 55/114

x là dấu nhân nha mn

a)17/6                                  b)5

c)25312                               d)101248

a,Ta có :

1/3 + 1/16 + 1/19 + 1/21 + 1/61 + 1/72 + 1/83 + 1/94 = 0,54

3/4 = 0,6

=>b < 3/4

=(8*7-8*2-8*5)*((1+9)+(2+8)+(3+7)+(4+6)+5)

=(8*(7-2-5))*(10+10+10+10+5)

=(8*0)*(10+10+10+10+5)

=0*(10+10+10+10+5)

=0

0 ko ns nhiều

Giải:

A=5/9+2/15-6/9

   =(5/9-6/9)+2/15

   = -1/9 + 2/15

   = 1/45

B=2/7-3/8+4/7+1/7-5/8+5/15

   = (2/7+4/7+1/7) + (-3/8-5/8) +1/3

   = 1+ (-1) +1/3

   =1/3

C=3/5+1/15+1/57+1/3-2/9-3/4-1/36

   =9/15+1/15+1/57+19/57-8/36-27/36-1/36

   =(9/15+1/15)+(1/57+19/57)+(-8/36-27/36-1/36)

   =2/3+20/57+(-1)

   =58/57+(-1)

   =1/57

D=1/1.2+1/2.3+1/3.4+...+1/99.100

   =1/1-1/2+1/2-1/3+1/3-1/4+...+1/99-1/100

   =1/1-1/100

   =99/100

Câu E mình ko biết làm nhé!

(8 x 7 - 16 - 5 x 8) x (1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9)

= [8 x 7 - 8 x 2 - 5 x 8] x [(1 + 9) + (2 + 8 ) + (3 + 7) + (4 + 6) + 5]

= [8 x (7 - 2 - 5)] x [10 + 10 + 10 + 10 + 10 + 5]

= (8 x 0) x 45

= 0

1 + 3 + 5 + 7 + 9 + 11 + 13 + 15 + 17 + 19

= (1 + 9) + (3 + 7) + (5 + 15) + (11 + 19) + (13 + 17)

= 10 + 10 + 20 + 30 + 30

= 100

b)

S2=6/2x5+6/5x8+6/8x11+...+6/29x32

=2.(3/2.5+3/5.8+...+3/29.32)

=2.(1/2-1/5+1/5-1/8+...+1/29-1/32)

=2.(1/2-1/32)

=2.15/32

=15/16

a)

Ta có:

S1=2/3x5+2/5x7+2/7x9+...+2/97x99

=1/3-1/5+1/5-1/7+...+1/97-1/99

=1/3-1/99

=32/99

\(a.\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+\frac{1}{4\cdot5}=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}\)

\(=\frac{1}{2}-\frac{1}{5}\)

\(=\frac{3}{10}\)

\(b.\frac{2}{2\cdot3}+\frac{2}{3\cdot4}+\frac{2}{4\cdot5}\)

\(=2\left(\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+\frac{1}{4\cdot5}\right)\)

\(=2\cdot\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}\right)\)

\(=2\cdot\left(\frac{1}{2}-\frac{1}{5}\right)\)

\(=2\cdot\frac{3}{10}=\frac{3}{5}\)

\(c.\frac{1}{2\cdot3}+\frac{2}{3\cdot5}+\frac{3}{5\cdot8}\)

\(=\frac{1}{6}+\frac{2}{15}+\frac{3}{40}\)

\(=\frac{3}{8}\)

k nha 500 AE

a, \(\frac{1}{2\times3}+\frac{1}{3\times4}+\frac{1}{4\times5}\)

\(=\frac{3-2}{2\times3}+\frac{4-3}{3\times4}+\frac{5-4}{4\times5}\)

\(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}\)

\(=\frac{1}{2}-\frac{1}{5}\)

\(=\frac{3}{10}\)

b, \(\frac{2}{2\times3}+\frac{2}{3\times4}+\frac{2}{4\times5}\)

\(=\frac{3-2}{2\times3}+\frac{4-3}{3\times4}+\frac{5-4}{4\times5}\)

\(=\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}\right)\times\frac{2}{1}\)

\(=\left(\frac{1}{2}-\frac{1}{5}\right)\times\frac{2}{1}\)

\(=\frac{3}{10}\times\frac{2}{1}\)

\(=\frac{3}{5}\)

c,  \(\frac{1}{2\times3}+\frac{2}{3\times5}+\frac{3}{5\times8}\)

 \(=\frac{3-2}{2\times3}+\frac{5-3}{3\times5}+\frac{8-5}{5\times8}\)

\(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}\)

\(=\frac{1}{2}-\frac{1}{8}\)

\(=\frac{3}{8}\)

=7/8.8/3.9/5.5

=7/3.9

=21

`7/8xx9/5xx8/3xx5=[7xx3xx3xx8xx5]/[8xx5xx3]=7xx3=21`