~ Bạn tham khảo ~
`A = 13/15 + 13/35 + 13/63 + 13/99`
`=> A = 2/2*(13/[3*5] + 13/[5*7] + 13/[7*9] + 13/[9*11])`
`=> A = 2/2*13(1/[3*5] + 1/[5*7] + 1/[7*9] + 1/[9*11])`
`=> A = 13/2*(2/[3*5] + 2/[5*7] + 2/[7*9] + 2/[9*11])`
`=> A = 13/2*(1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11)`
`=> A = 13/2*(1/3-1/11)`
`=> A = 13/2*8/33`
`=> A = 52/33`