So sánh phân số:
A=\(\dfrac{10^{2001}+1}{-10^{2002}+1}\)và B= \(\dfrac{-10^{2002}+1}{10^{2003}+1}\)
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\(A=\dfrac{10^{2001}+1}{10^{2002}+1}\Leftrightarrow10A=\dfrac{10^{2002}+10}{10^{2002}+1}=1+\dfrac{9}{10^{2002}+1}\)
\(B=\dfrac{10^{2002}+1}{10^{2003}+1}\Leftrightarrow10B=\dfrac{10^{2003}+10}{10^{2003}+1}=1+\dfrac{9}{10^{2003}+1}\)
Từ đó suy ra \(10A>10B\) hay \(A>B\)
Áp dụng bất đẳng thức :\(\dfrac{a}{b}< 1\Leftrightarrow\dfrac{a}{b}< \dfrac{a+m}{b+m}\) ta có :
\(B=\dfrac{10^{2002}+1}{10^{2003}+1}< \dfrac{10^{2002}+1+9}{10^{2003}+1+9}=\dfrac{10^{2002}+10}{10^{2003}+10}=\dfrac{10\left(10^{2001}+1\right)}{10\left(10^{2002}+1\right)}=\dfrac{10^{2001}+1}{20^{2002}+1}=A\)
\(\Leftrightarrow A>B\)
Ta có: 10 *(10^2001+1)/10^2002+1 = 10^2002+10/10^2002+1 = (10^2002+1)+9/10^2002+1 = 1+9/10^2002+1
10*(10^2002+1)/10^2003+1 = 10^2003+10/10^2003+1 = (10^2003+1)+9/10^2003+1 = 1+9/10^2003+1
Vì 9/10^2002+1>9/10^2003+1 nên 1+9/10^2002+1>1+9/10^2003+1
Vậy: 10^2001+1/10^2002+1>10^2002+1/10^2003+1
ta thấy:
\(B< 1\Rightarrow B< \frac{10^{2002}+1+9}{10^{2003}+1+9}=\frac{10^{2002}+10}{10^{2003}+10}=\frac{10\left(10^{2001}+1\right)}{10\left(10^{2002}+1\right)}=\frac{10^{2001}+1}{10^{2002}+1}=A\)
=>B<A
vậy.......
Ta có:
\(A=\frac{10^{2001}+1}{10^{2002}+1}\Rightarrow10A=\frac{10\left(10^{2001}+1\right)}{10^{2002}+1}=\frac{10^{2002}+10}{10^{2002}+1}=\frac{10^{2002}+1+9}{10^{2002}+1}=1+\frac{9}{10^{2002}+1}\)
\(B=\frac{10^{2002}+1}{10^{2003}+1}\Rightarrow10B=\frac{10\left(10^{2002}+1\right)}{10^{2003}+1}=\frac{10^{2003}+10}{10^{2003}+1}=\frac{10^{2003}+1+9}{10^{2003}+1}=1+\frac{9}{10^{2003}+1}\)
Vì \(\frac{9}{10^{2002}+1}>\frac{9}{2^{2003}+1}\Rightarrow1+\frac{9}{10^{2002}+1}>1+\frac{9}{2^{2003}+1}\Rightarrow10A>10B\Rightarrow A>B\)
Vậy A > B
Tham khảo:Câu hỏi của Trần Trí Trung - Toán lớp 6 - Học toán với OnlineMath
\(A=\frac{10^{2001}+1}{10^{2002}+1}=\frac{\left(10^{2001}+1\right)\left(10^{2003}+1\right)}{\left(10^{2002}+1\right)\left(10^{2003}+1\right)}=\frac{10^{4004}+10^{2001}+10^{2003}+1}{\left(10^{2002}+1\right)\left(10^{2003}+1\right)}\)
\(B=\frac{10^{2002}+1}{10^{2003}+1}=\frac{\left(10^{2002}+1\right)\left(10^{2002}+1\right)}{\left(10^{2003}+1\right)\left(10^{2002}+1\right)}=\frac{10^{4004}+2.10^{2002}+1}{\left(10^{2003}+1\right)\left(10^{2002}+1\right)}\)
Vì 102001 + 102003 < 2.102002 nên A < B
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