Đáp án:
B = 1/3 + 1/6 + 1/10 + ... + 1/ X x ( X + 1 ) = 2009 / 2011
=>2/6+2/12+2/20+.....+2/x(x+1)=2019/2011
=>2{(1/6+1/12+1/20+.....+2/x(x+1)}=2019/2011
=>2{1/2.3+1/3.4+1/4.5+.....+1/x(x+1)}=2019/2011
=>2{1/2-1/3+1/3-1/4+1/4-1/5+.....+1/x-1/(x+1)}=2019/2011
=>2.{1/2-1/x+1}=2019/2011=>1/2+1/x+1=2009/4022=>1/x+1=1/2011=>x+1=2011=>x=2010
Vậyx=2010
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