cần gấp ..... .....................

`log_5 \ (sqrt2)/(sqrt15)`

`=log_5 sqrt2 - log_5 sqrt15`

`=1/2log_5 2 - 1/2log_5 15`

`=1/2a-1/2log_5 (3*5)`

`=1/2a-1/2log_5 3 -1/2log_5 5`

`=1/2a-1/2b-1/2`

`=(a-b-1)/2`

`=>` Chọn `D`