\begin{array}{c|c|c}\color{gainsboro}{𝕻𝖍}\color{lightgrey}{𝖚𝖔}\color{silver}{𝖓𝖌}\color{darkgray}{𝕿𝖍}\color{gray}{𝖆}\color{dimgray}{𝖓}\color{black}{𝖍}\end{array}

Ta có: `A=1/(1xx3)+1/(3xx5)+1/(5xx7)+...+1/(143xx145)`

`2A=2/(1xx3)+2/(3xx5)+2/(5xx7)+...+2/(143xx145)`

`2A=1-1/3+1/3-1/5+1/5-1/7+...+1/143-1/145`

`2A=1-1/145`

`2A=144/145`

`A=144/145:2`

`A=72/145`