Question

Tính bằng cách thuận tiện nhất

Answer 1

`1/( 1xx3 ) + 1/( 3xx5 ) + 1/( 5 xx 7 ) + 1/( 7 xx 9 )`

`= 1/2 xx ( 2/( 1xx3 ) + 2/( 3xx5 ) + 2/( 5 xx 7 ) + 2/( 7 xx 9 ))`

`= 1/2 xx ( 1 - 1/3 + 1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 )`

`= 1/2 xx ( 1 - 1/9 )`

`= 4/9` 

Answer 2

   `1/[1xx3]+1/[3xx5]+1/[5xx7]+1/[7xx9]`

`=1/2xx(2/[1xx3]+2/[3xx5]+2/[5xx7]+2/[7xx9])`

`=1/2xx(1-1/3+1/3-1/5+1/5-1/7+1/7-1/9)`

`=1/2xx(1-1/9)`

`=1/2xx(9/9-1/9)`

`=1/2xx8/9`

`=4/9`