`M = 1/90 - 1/72 - 1/56 - 1/42 - 1/30 - 1/20 - 1/12 - 1/6 - 1/2`
`M = 1/90 - (1/72 + 1/56 + 1/42 + 1/30 + 1/20 + 1/12 + 1/6 + 1/2)`
`M = 1/90 - (1/{9.8} + 1/{8.7} + 1/{7.6} + 1/{6.5} + 1/{5.4} + 1/{4.3} + 1/{3.2} + 1/{2.1})`
`M = 1/90 - (1/{1.2} + 1/{2.3} + 1/{3.4} + 1/{4.5} + 1/{5.6} + 1/{6.7} + 1/{7.8} + 1/{8.9})`
`M = 1/90 - (1 - 1/9)`
`M = 1/90 - 8/9`
`M = -79/90`
$\text{Vậy}$ `M = -79/90`