Giải thích các bước giải:

`( y - 1/2 ) : ( 1/2 + 1/6 + 1/12 + 1/20 + ... + 1/90 ) = 1/3`

`( y - 1/2) : ( 1/(1 . 2 ) + 1/(2. 3 ) + 1/(3 .4) + 1/( 4 .5 ) + ... + 1/(9 . 10) ) = 1/3`

`( y - 1/2 ) : ( 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + ... + 1/9 - 1/10 ) = 1/3`

`( y - 1/2 ) : ( 1 - 1/10 ) = 1/3`

`( y - 1/2 ) : ( 10/10 - 1/10 ) = 1/3`

`( y - 1/2 ) : 9/10 = 1/3`

`y - 1/2 = 1/3 . 9/10`

`y - 1/2 = 3/10`

`y = 3/10 + 1/2`

`y = 3/10 + 5/10`

`y = 8/10`

`y = 4/5`

Vậy, `y = 4/5`

`#By`