`D = 1/(1 X 4) + 1/(4 X 7) + 1/(7 X 10) + ... + 1/(47 X 50)`
`⇒ D = 1/3 X ( 3/(1 X 4) + 3/(4 X 7) + 3/(7 X 10) + ... + 3/(47 X 50) )`
`⇒ D = 1/3 X ( 1 - 1/4 + 1/4 - 1/7 + 1/7 - 1/10 + ... + 1/47 - 1/50 )`
`⇒ D = 1/3 X ( 1 - 1/50 )`
`⇒ D = 1/3 X 49/50`
`⇒ D = 49/150`
Vậy `D = 49/150`