Ta có: a= 2009 x 2010 = 2009 x (2010+1)=2009 x 2010 + 2009
b= 2010 x 2010= 2010 x (2009+1)=2010 x 2009 + 2010
=> a < b (Vì 2009 x 2010 + 2009 < 2010 x 2009 + 2010)
Vậy a < b
a=2009.2011=4040099
b=2010.2010=4040100
Vi 4040099<4040100
nen a < b
Vay a < b .
a=2009(2010+1)=2009.2010+2009
b=2010.2010=2010(2009+1)=2010.2009+2010
vậy a<b