5A=(1/5+1/25+1/125+1/625+1/3125)*5
5A=(1+1/5+1/25+1/125+1/625)
5A-A=(1+1/5+1/25+1/125+1/625)-(1/5+1/25+1/125+1/625+1/3125)=1-1/3125
4A=1-1/3125=3124/3125
A=3124/3125:4=781/3125.
`=>` Ta có:
`5A=(1/5+1/25+1/125+1/625+1/3125) xx 5`
`5A=(1+1/5+1/25+1/125+1/625)`
`5A-A=(1+1/5+1/25+1/125+1/625)-(1/5+1/25+1/125+1/625+1/3125)=1-1/3125`
`4A=1-1/3125=3124/3125
A=3124/3125:4=781/3125.