a) A = 1/1.2+1/2.3+1/3.4+...+1/2009.2010
= 1-1/2010
= 2009/2010
b) B= 2/1.3+2/3.5+2/5+...+2/2010.2012
= 1 - 1/2012
= 2011/2012
Bài 7
1/3 + 1/6+1/10+,,,+2/x(x+1) = 4020/2011
=> 2 ( 1/6+1/10+...+1/x(x+1)=4020/2011
=> 1/6+1/10+...+1/x(x+1)= 4020/2011:2=2020/2021
=> 1-1/x+1 = 2020/2021
=> 1/x+1 = 1 - 2020/2021
=> 1/x+1= 1/2021
=> x +1=2021
=> x = 2020
Bài 8
1/2010-1/2010.2009-1/2009.2008-...-1/3.2-1/2.1
= 1/2010 - ( 1-1/2010)
= 1/2010 - 2009/2010
= ( -1004/1005)
Đúng 3
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