Ta có:A=1/19+9/19*29+9/29*39+...+9/1999*2009
A=9/9*19+9/19*29+9/29*39+...+9/1999*2009
A=9/10*(10/9*19+10/19*29+10/29*39+...+10/1999*2009)
A=9/10*(1/9-1/19+1/19-1/29+1/29-1/39+...+1/1999-1/2009)
A=9/10*(1/9-1/2009)
A=9/10*2000/18081
A=200/2009(không chắc nha)