Đặt `A=5/3.13 + 5/13.23 + 5/23.33 +...+5/83.93`
`2A = 2(5/3.13 + 5/13.23 + 5/23.33 +...+5/83.93)`
`2A= 10/3.13 + 10/13.23 + 10/23.33 +...+10/83.93`
`2A= 1/3 - 1/13 + 1/13 - 1/23 +....+1/83 - 1/93`
`2A= 1/3 - 1/93`
`2A=10/31`
`A= 10/31 : 2`
`A= 10/31 . 1/2`
`A= 5/31`