x1 + x2 + x3 + ..... + x2011 = 0
x1 + x2 = x3 + x4 = .... = x2009 + x2010 = 2
(x1 + x2) + (x3 + x4) + ...... + (x2009 + x2010) + x2011 = 0
2 + 2 + 2 + ... + 2 + x2011 = 0
2 x 1005 + x2011 = 0
2010 + x2011 = 0
x2011 = 0 - 2010
x2011 = -2010
(x1+x2) + (x3+x4)+..........+ (x2009+x2010)+x2011 = 2+2+............+2+x2011 = 2. 2005 + x2011 = 2010 + x2011 = 0 => x2011 = 0-2010 = -2011
vậy x2011 là -2010