Giúp em với Anh chị ơi

`a.P=78xx31+78xx24+78xx17+22xx72`

`P=78xx(31+24+17)+22xx72`

`P=78xx72+22xx72`

`P=72xx(78+22)`

`P=72xx100`

`P=7200`

`-` Vậy : `P=7200.`

`···········` 

`b.S=1+1/3+1/9+1/27+...+1/2187`

`3xxS=3+1+1/3+1/9+....+1/729`

`3xxS-S=(3+1+1/3+1/9+...+1/729)-(1+1/3+1/9+1/27+...+1/2187)`

`Sxx(3-1)=3-1/2187`

`2xxS=6561/2187-1/2187`

`2xxS=6560/2187`

`S=6560/2187:2`

`S=6560/4374`

`S=3280/2187`

`-` Vậy : `S=3280/2187` 

`#Sal_tSea`