-x ( 1/1x2 + 1/ 2x3 + ..... 1/ 2011x2012 ) = 2011/2012
1/ 1x2 = 1/1 - 1/2
1/2x3 = 1/2 - 1/3
....
obtinem -x ( 1/1 -1/2 +1/2 - 1/3 + 1/3 - ..........1/2011 - 1/2012 ) = 2011 / 2012
reducem -1/2 cu 1/2 , etc
rezulta
-x ( 1/1 - 1/2012 ) = 2011 /2012
-x ( 2011 /2012 ) = 2011 / 2012
x= -1