`H = 5/(2. 1) + 4/(1. 11) + 3/(11. 2) + 1/(2.15) + 13/(15. 4)`
`= 7. 1/7. (5/(2. 1) + 4/(1. 11) + 3/(11. 2) + 1/(2.15) + 13/(15. 4))`
`= 7. (5/(2. 1. 7) + 4/(1. 7. 11) + 3/(11. 2. 7) + 1/(2. 7. 15) + 13/(15. 4. 7))`
`= 7. (5/(2. 7) + 4/(7. 11) + 3/(11. 14) + 1/(14. 15) + 13/(15. 28))`
`= 7. (1/2 - 1/7 + 1/7- 1/11 + 1/11 - 1/14 + 1/14 - 1/15 + 1/15 - 1/28)`
`= 7. (1/2 - 1/28)`
`= 7. 1/2 - 7. 1/28`
`= 7/2 - 1/4`
`= 14/4 - 1/4`
`= 13/4`
Vậy `H = 13/4`