~ Bạn tham khảo ~
`C= 1/15 + 1/35 + 1/63 + 1/99 + 1/143`
`= 2/2*(1/[3*5] + 1/[5*7] + 1/[7*9] + 1/[9*11] + 1/[11*13]`
`= 1/2*(2/[3*5] + 2/[5*7] + 2/[7*9] + 2/[9*11] + 2/[11*13])`
`= 1/2*(1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11+1/11-1/13)`
`= 1/2*(1/3-1/13)`
`= 1/2*10/39`
`= 5/39`