Question

đáp án thôi ạ

Answer 1

`S = 1/( 5 xx 9 ) + 1/( 9xx13 ) +.....+1/( 41xx45)`

   `= 1/4 xx ( 1/( 5 xx 9 ) + 1/( 9xx13 ) +.....+1/( 41xx45))`

   `= 1/4 xx ( 1/5 - 1/9 + 1/9 - 1/13 +....+1/41-1/45)`

   `= 1/4 xx ( 1/5 - 1/45 )`

   `= 2/45`

Answer 2

`4S = 1/(5xx9) + 1/(9xx13) + ... + 4/(41xx45)`

`4S = 1/5 - 1/9 + 1/9 - 1/13 + ... + 1/41 - 1/45`

`4S = 8/45`

`=> S = 2/45`

Answer 3

Answer 4

4S  = \( (\dfrac{1}{5x9})\)+\( (\dfrac{1}{9x13})\)+\( (\dfrac{1}{13x17})\)+....\( (\dfrac{1}{41x45})\)

4S =  [\( (\dfrac{1}{5x9})+(\dfrac{1}{9x13})+(\dfrac{1}{13x17})+...+(\dfrac{1}{41x45})\)]

4S = \((\dfrac{1}{5}-\dfrac{1}{9}+\dfrac{1}{ 9}-\dfrac{1}{ 13}+...+\dfrac{1}{ 41}-\dfrac{1}{ 45}\))

4S =  (\(\dfrac{1}{ 5}-\dfrac{1}{ 45}\))

4S =  \(\dfrac{8}{ 45}\)

S = \(​​\dfrac{2}{45}\)

S = \(\dfrac{32}{ 45}\)

 

Answer 5

Toán lớp 4 ????