Đáp án:
`1/(1 + 2015/2017 + 2015/2019) + 1/(1 + 2017/2015 + 2017/2019) + 1/(1 + 2019/2015 + 2019/2017)`
`=1/(2015/2015 + 2015/2017 + 2015/2019) + 1/(2017/2015 + 2017/2017 + 2019/2019) + 1/(2019/2015+ 2019/2017 + 2019/2019)`
`=1/(2015 . ( 1/2015 + 1/2017 + 1/2019) ) + 1/(2017 . (1/2015 + 1/2017 + 1/2019) ) + 1/(2019. (1/2015 + 1/2017 + 1/2019) )`
`=(1/2015)/(1/2015 + 1/2017 + 1/2019) + (1/2017)/(1/2015 + 1/2017 + 1/2019) + (1/2019)/(1/2015 + 1/2017 + 1/2019)`
`=(1/2015 + 1/2017 + 1/2019)/(1/2015 + 1/2017 + 1/2019)`
`=1`
`text{ @toanisthebest}`
