Bài 5 :
`a) S = 5^2/( 1 . 6 ) + 5^2/( 6 . 11 ) + 5^2/( 11 . 16 ) + 5^2/( 16 . 21 ) + 5^2/( 21 . 26 )`
`S = 5 . ( 5/( 1 . 6 ) + 5/( 6 . 11 ) + 5/( 11 . 16 ) + 5/( 16 . 21 ) + 5/( 21 . 26 ) )`
`S = 5 . ( 1 - 1/6 + 1/6 - 1/11 + 1/11 - 1/16 + 1/16 - 1/21 + 1/21 - 1/26 )`
`S = 5 . ( 1 - 1/26 )`
`S = 5 . ( 26/26 - 1/26 )`
`S = 5 . 25/26`
`S = 125/26`