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So sánh A và B
\(A=\frac{100^{2015}+1}{100^{2014}+1}\)
\(B=\frac{100^{2016}+1}{100^{2015}+1}\)
Gấp nha!
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có:
B>\(\frac{100^{2016}+1+99}{100^{2015}+1+99}\)=\(\frac{100^{2016}+100}{100^{2015}+100}\)=\(\frac{100\left(100^{2016}+1\right)}{100\left(100^{2015}+1\right)}\)=\(\frac{100^{2015}+1}{100^{2014}+1}\)=A
Vậy B>A
![](https://rs.olm.vn/images/avt/0.png?1311)
a,\(A=\frac{1}{5}+\frac{1}{5^2}+\frac{1}{5^3}+...+\frac{1}{5^{100}}\)
\(=>5A=1+\frac{1}{5}+\frac{1}{5^2}+...+\frac{1}{5^{99}}\)
\(=>5A-A=1-\frac{1}{5^{100}}=>A=\frac{1-\frac{1}{5^{100}}}{4}\)
b, Ta có \(1-\frac{1}{5^{100}}< 1=>\frac{1-\frac{1}{5^{100}}}{4}< \frac{1}{4}\)hay \(A< \frac{1}{4}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(A=\frac{2014}{2015}+\frac{2015}{2016}>\frac{2014}{2016}+\frac{2015}{2016}>\frac{2014+1015}{2015+2016}=B\Rightarrow A>B\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có:
A=100^2015+1/100^2016+1 suy ra 100A=100^2016+100/100^2016+1=100^2016+1+99/100^2016+1=1/99/100^2016+1
Lại có
B=100^2016+1/100^2017+1 suy ra 100B=100^2017+100/100^2017+1=100^2017+1+99/100^2017+1=1/99/100^2017+1
Vì1/99/100^2016+1>1/99/100^2017+1 suy ra A>B
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có:
A=1/3 - 2/3^2+3/3^3 - 4/3^4+ ... - 100/3^100
=>3A=1 -2/3 +3/3^2 - 4/3^3+ ... - 100/3^99
=>4A=A+3A=1-1/3+1/3^2-1/3^3+...-1/3^99 - 100/3^100
=>12A=3.4A=3-1+1/3-1/3^2+...-1/3^98 - 100/3^99
=>16A=12A+4A=3-1/3^99-100/3^99-100/3^1...
<=>16A=3-101/3^99-100/3^100
<=>A=3/16-(101/3^99+100/3^100)/16 < 3/16
Suy ra A<3/16
\(A=\frac{100^{2016}+1}{100^{2015}-1}\)
\(\frac{1}{100}.A=\frac{100^{2016}+1}{100\left(100^{2015}-1\right)}\)
\(=\frac{100^{2016}+1}{100^{2016}-100}\)
\(=\frac{\left(100^{2016}-100\right)+101}{100^{2016}-100}\)
\(=\frac{100^{2016}-100}{100^{2016}-100}\)\(+\frac{101}{100^{2016}-100}\)
\(=1+\frac{101}{100^{2016}-100}\)
\(B=\frac{100^{2015}+1}{100^{2014}-1}\)
\(\frac{1}{100}.B=\frac{100^{2015}+1}{100\left(100^{2014}-1\right)}\)
\(=\frac{100^{2015}+1}{100^{2015}-100}\)
\(=\frac{\left(100^{2015}-100\right)+101}{100^{2015}-100}\)
\(=\frac{100^{2015}-100}{100^{2015}-100}\)\(+\frac{101}{100^{2015}-100}\)
\(=1+\frac{101}{100^{2015}-100}\)
\(\hept{\begin{cases}Vì101>0\\100^{2016}-100>100^{2015}-100>0\end{cases}}\)
\(\Rightarrow\frac{101}{100^{2016}-100}< \frac{101}{100^{2015}-100}\)
\(\Rightarrow1+\frac{101}{100^{2016}-100}< 1+\frac{101}{100^{2015}-100}\)
\(\Rightarrow\frac{1}{100}.A< \frac{1}{100}.B\)
\(\Rightarrow A< B\left(vì\frac{1}{100}>0\right)\)
Vậy A<B
cảm ơn cậu nhé!