Sửa đề :
`1/4+1/12+1/36+...+1/972`
Đặt `A=1/4+1/12+1/36+...+1/972`
`3A=3(1/4+1/12+1/36+...+1/972)`
`3A=3/4+3/12+3/36+...+3/972`
`2A=3A-A=(3/4+3/12+3/36+...+3/972)-(1/4+1/12+1/36+...+1/972`
`2A=3/4-1/972`
`2A=182/243`
`A=364/243`
Vậy `1/4+1/12+1/36+...+1/972=364/243`