giúp vứi =( càng lẹ càng tốt

Ta có :

`M = 1/(1.5) + 2/(5.13) + 3/(13.25) + 4/(25.41)`

`=> M = (1.4)/(1.5.4) + (2.4)/(5.13.4) + (3.4)/(13.25.4) + (4.4)/(25.41.4)`

`=> M = 1/4 . (4/(1.5) + 8/(5.13) + 12/(13.25) + 16/(25.41))`

`=> M = 1/4 . (1/1 - 1/5 + 1/5 - 1/13 + 1/13 - 1/25 + 1/25 - 1/41)`

`=> M = 1/4 . (1/1 - 1/41)`

`=> M = 1/4 . 40/41`

`=> M = 10/41 = 20/81`

Lại có :

`N = 2/(1.7) + 3/(7.16) + 4/(16.28) + 5/(28.43) + 6/(43.61)`

`=> N = (2.3)/(1.7.3) + (3.3)/(7.16.3) + (4.3)/(16.28.3) + (5.3)/(28.43.3) + (6.3)/(43.61.3)`

`=> N = 1/3 . (6/(1.7) + 9/(7.16) + 12/(16.28) + 15/(28 . 43) + 18/(43.61))`

`=> N = 1/3 . (1/1 - 1/7 + 1/7 - 1/16 + 1/16 - 1/28 + 1/28 - 1/43 + 1/43 - 1/61)`

`=> N = 1/3 . (1/1 - 1/61)`

`=> N = 1/3 . 60/61`

`=> N = 20/61`

Vì `20/81 < 20/61` nên `M < N`

Vậy `M < N`

`#dtkc`