`2/( 1 . 2 ) + 2/( 2 . 3 ) + 2/( 3 . 4 ) + 2/( 4 . 5 ) + 2/( 5 . 6 )`
`= 2 . ( 1/( 1 . 2 ) + 1/( 2 . 3 ) + 1/( 3 . 4 ) + 1/( 4 . 5 ) + 1/( 5 . 6 ) )`
`= 2 . ( 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 )`
`= 2 . ( 1 - 1/6 )`
`= 2 . 5/6`
`= 5/3`