$\text {·Tính nhanh:}$
`M= 1/4+1/(10)+1/(18)+1/(40)+1/(54)+1/(70)`
`M= (1/4+1/(40))+(1/(10)+1/(70))+(1/(18)+1/(54))`
`M= ((10)/(40)+1/(40))+(7/(70)+1/(70))+(3/(54)+1/(54))`
`M= (11)/(40)+8/(70)+4/(54)`
`M= (11)/(40)+4/(35)+2/(27)`
`M= (10395)/(37800)+(4320)/(37800)+(2800)/(37800)`
`M= (17515)/(37800)`
`M= (3503)/(7560)`
$\text {@lamtung2007}$