`M= 1/3.7 + 1/7.11 + 1/11.15+1/15.19+ 1/19.23`
`4M = 4( 1/3.7+ 1/7.11 + 1/11.15+ 1/15.19+1/19.23)`
`4M = 4/3.7 + 4/7.11 + 4/11.15 +4/15.19+4/19.23`
`4M = 1/3 - 1/7 + 1/7- 1/11 + 1/11 -1/15+1/15 - 1/19+1/19-1/23`
`4M = 1/3 - 1/23`
`4M =20/69`
`M= 20/69 : 4`
`M= 20/69 . 1/4`
`M= 5/69`
Vậy `M= 5/69`