X 、y互为相反数,则x+y=0
（2x+y)(x+2y)与1/2011的相反数相等,则(2x+y)(x+2y)=-1/2011
则(x+x+y)(x+y+y)=-1/2011
(x+0)(0+y)=-1/2011
所以xy=-1/2011