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Đặt A=\(\frac{10^{2006}+1}{10^{2007}+1}\);\(B=\frac{10^{2007}+1}{10^{2008}+1}\)
10A=\(\frac{10\left(10^{2006}+1\right)}{10^{2007}+1}\)=\(\frac{10^{2007}+1+9}{10^{2007}+1}\)
10B=\(\frac{10\left(10^{2007}+1\right)}{10^{2008}+1}=\frac{10^{2008}+1+9}{10^{2008}+1}\)
Vì \(\frac{9}{10^{2007}+1}>\frac{9}{10^{2008}+1}\)nên 10A>10B nên A>B
Đặt A=1+2-3-4+5-6-7-8+9+10-...+2006-2007-2008+2009
Ta có:A=1+2-3-4+5+6-7-8+9+10-...+2006-2007-2008+2009
A=1+(2-3-4+5)+(6-7-8+9)+...+(2006-2007-2008+2009)
A=1+0+0+0+....+0
A=1
Ta có: A=\(\frac{10^{2006}+1}{10^{2007}+1}\)
=>10A=\(\frac{10\left(10^{2006}+1\right)}{10^{2007}+1}=\frac{10^{2007}+10}{10^{2007}+1}=1+\frac{9}{10^{2007}+1}\)
Ta có: B=\(\frac{10^{2007}+1}{10^{2008}+1}\)
=>10B=\(\frac{10\left(10^{2007}+1\right)}{10^{2008}+1}=\frac{10^{2008}+10}{10^{2008}+1}=1+\frac{9}{10^{2008}+1}\)
Mà \(\frac{9}{10^{2007}+1}>\frac{9}{10^{2008}+1}\) (do 102007+1<102008+1)
=>\(1+\frac{9}{10^{2007}+1}>1+\frac{9}{10^{2008}+1}\)
=>10A>10B
=>A>B
Áp dụng a/b < 1 => a/b < a+m/b+m (a,b,m thuộc N*)
=> \(B=\frac{10^{2007}+1}{10^{2008}+1}< \frac{10^{2007}+1+9}{10^{2008}+1+9}\)
=> \(B< \frac{10^{2007}+10}{10^{2008}+10}\)
=> \(B< \frac{10.\left(10^{2006}+1\right)}{10.\left(10^{2007}+1\right)}\)
=> \(B< \frac{10^{2006}+1}{10^{2007}+1}=A\)
= 1+(2-3-4+5)+(6-7-8+9)+ ... +(2006-2007-2008+2009)
=1+0+0+ ... +0
=1
yêu cầu so sánh 2 phân số
Đặt \(A=\frac{10^{2006}+9}{10^{2007}+9}\)
\(\Rightarrow10A=\frac{10^{2007}+90}{10^{2007}+9}=1+\frac{81}{10^{2007}+9}\)
\(\frac{10^{2007}+9}{10^{2008}+9}=B\)
\(\Rightarrow10B=\frac{10^{2008}+90}{10^{2008}+9}=1+\frac{81}{10^{2008}+9}\)
Vì\(10A>10B\Rightarrow A>B\)