$Đáp$ $án$ $+$ $Giải$ $thích$ $các$ $bước$ $giải$ $:$

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  `{a-3}/{a+3} = {b-6}/{b+6}`

`=>` `( a - 3 )( b + 6 ) = ( a + 3 )( b - 6 )`

`=>` `ab + 6a - 3b - 18 = ab - 6a + 3b - 18`

`=>` `12a = 6b`

`=>` `a/6 = b/12`

`=>` `a/b = 6/12`

`=>` `a/b = 1/2` ( đpcm )