`=> ((x + 5)/2005 + 1) + ((x + 6)/2004 + 1) + ((x + 7)/2003 + 1) = 0`
`=> (x + 2010)/2005 + (x + 2010)/2004 + (x + 2010)/2003 = 0`
`=> (x + 2010) . (1/2005 + 1/2004 + 1/2003) = 0`
`=> x + 2010 = 0` (vì `1/2005 + 1/2004 + 1/2003 > 0`)
`=> x = 0 - 2010`
`=> x = -2010`