`9)` Ta có `4a-1/4a=2025`
`axx(4-1/4)=2025`
`axx15/4=2025`
`a=2025:15/4`
`a=540`
Vậy `a=540`.
`10)`
`A=(1/15+1/35+1/63+1/99)xxa=8/11`
`2A=2xx(1/(3xx5)+1/(5xx7)+1/(7xx9)+1/(9xx11))xxa=8/11`
`2A=(2/(3xx5)+2/(5xx7)+2/(7xx9)+2/(9xx11))xxa=8/11`
`2A=(1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11)xxa=8/11`
`2A=(1/3-1/11)xxa=8/11`
`2A=8/33xxa=8/11`
`A=8/33:2xxa=8/11`
`A=4/33xxa=8/11`
`4/33xxa=8/11`
`a=8/11:4/33`
`a=6`
Vậy `a=6`.