đề khó ! đề khó

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L

locxoaymgk

viết lại này:
[TEX]A=[/TEX][TEX]\frac{1}{1.3}+[/TEX][TEX]\frac{1}{3.5}+................+[/TEX][TEX]\frac{1}{2009.2011}[/TEX]
[TEX] 2A=[/TEX][TEX]\frac{2}{1.3}+[/TEX][TEX]\frac{2}{3.5}+................+[/TEX][TEX]\frac{2}{2009.2011}[/TEX]
[TEX]2A=[/TEX][TEX]\frac{1}{1}-[/TEX][TEX]\frac{1}{3}+[/TEX][TEX]\frac{1}{3}-[/TEX][TEX]\frac{1}{5}+.........+[/TEX][TEX]\frac{1}{2009}-[/TEX][TEX]\frac{1}{2011}[/TEX]
2A=[TEX]1-[/TEX][TEX]\frac{1}{2011}[/TEX]
[TEX]A=[/TEX] 2010/2011 : 2 = ........
 
T

thiennhan12

giải

1/1.3 + 1/3.5 + 1/5.7 +...+1/2009.2011
\Rightarrow 1/1 - 1/3 + 1/3 - 1/5 + 1/5 - 1/7+...+ 1/2099 - 1/2011
= 1/1 - 1/2011
= 2010/2011
Nhớ thanks mình nha
 
M

muttay04

Phạm Sư Mạnh alaways win!!!!!!!!

1/1.3 + 1/3.5 + 1/5.7 +...+1/2009.2011
\Rightarrow 1/1 - 1/3 + 1/3 - 1/5 + 1/5 - 1/7+...+ 1/2099 - 1/2011
= 1/1 - 1/2011
= 2010/2011
Nhớ thanks mình nha
thiennhan oi ban quen ko nhan voi 1/2 a
phai lam nhu nay
1/1.3 + 1/3.5 + 1/5.7 +...+1/2009.2011
=1/2.(2/1.3+2/3.5+.....+2/2009.2011)
=1/2.(1/1 - 1/3 + 1/3 - 1/5 + 1/5 - 1/7+...+ 1/2099 - 1/2011)
=1/2.(1/1-1/2011)
=1/2.2010/2011=1005/2011
nho thanks minh nha
 
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