So Sánh \(\sqrt{2016}-\sqrt{2015}\) Và \(\sqrt{2015}-\sqrt{2014}\)
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Ta có: \(\sqrt{2015}-\sqrt{2014}=\dfrac{2015-2014}{\sqrt{2015}+\sqrt{2014}}>\dfrac{2016-2015}{\sqrt{2016}+\sqrt{2015}}=\sqrt{2016}-\sqrt{2015}\)
Ta có: √2015−√2014=2015−2014√2015+√2014>2016−2015√2016+√2015=√2016−√2015
Áp dụng bđt \(\frac{\sqrt{a}+\sqrt{b}}{2}< \sqrt{\frac{a+b}{2}}\) với a > 0; b > 0; a \(\ne\) b ta có:
\(\frac{\sqrt{2016}+\sqrt{2014}}{2}< \sqrt{\frac{2016+2014}{2}}\)
\(\Rightarrow\frac{\sqrt{2016}+\sqrt{2014}}{2}< \sqrt{\frac{4030}{2}}\)
\(\Rightarrow\frac{\sqrt{2016}+\sqrt{2014}}{2}< \sqrt{2015}\)
\(\Rightarrow\sqrt{2016}+\sqrt{2014}< 2.\sqrt{2015}\)
\(\Rightarrow\sqrt{2016}-\sqrt{2015}< \sqrt{2015}-\sqrt{2014}\)
\(\sqrt{2016}-\sqrt{2015}=\dfrac{1}{\sqrt{2016}+\sqrt{2015}}\)
\(\sqrt{2015}-\sqrt{2014}=\dfrac{1}{\sqrt{2015}+\sqrt{2014}}\)
mà \(\sqrt{2016}+\sqrt{2015}>\sqrt{2014}+\sqrt{2015}\)
nên \(\sqrt{2016}-\sqrt{2015}< \sqrt{2015}-\sqrt{2014}\)
Áp dụng bđt \(\frac{\sqrt{a}+\sqrt{b}}{2}< \sqrt{\frac{a+b}{2}}\) với a > 0; b > 0; a khác b ta có:
\(\frac{\sqrt{2016}+\sqrt{2014}}{2}< \sqrt{\frac{2016+2014}{2}}\)
\(\Rightarrow\frac{\sqrt{2016}+\sqrt{2014}}{2}< \sqrt{\frac{4030}{2}}\)
\(\Rightarrow\sqrt{2016}+\sqrt{2014}< \sqrt{2015}.2\)
\(\Rightarrow\sqrt{2016}-\sqrt{2015}< \sqrt{2015}-\sqrt{2014}\)
A = \(\frac{2016-2015}{\sqrt{2016}+\sqrt{2015}}=\frac{1}{\sqrt{2016}+\sqrt{2015}}\); B = \(\frac{2015-2014}{\sqrt{2015}+\sqrt{2014}}=\frac{1}{\sqrt{2015}+\sqrt{2014}}\)
Mà \(\sqrt{2016}+\sqrt{2015}>\sqrt{2015}+\sqrt{2014}\) ( Vì \(\sqrt{2016}>\sqrt{2014}\))
Nên \(\frac{1}{\sqrt{2016}+\sqrt{2015}}
Ta đặt \(x=2015\), khi đó \(2014=x-1,2016=x+1.\) Ta có như sau
\(2014^2\cdot2016=\left(x-1\right)^2\left(x+1\right)=\left(x^2-1\right)\left(x-1\right)\)\(
Có: \(\sqrt{2015}< \sqrt{2016}\)
=>\(\frac{1}{\sqrt{2015}}>\frac{1}{\sqrt{2016}}\)
=>\(\frac{1}{\sqrt{2015}}-\frac{1}{\sqrt{2016}}>0\)
=>\(\sqrt{2015}+\sqrt{2016}+\frac{1}{\sqrt{2015}}-\frac{1}{\sqrt{2016}}>\sqrt{2015}+\sqrt{2016}\)
=>\(\left(\sqrt{2015}+\frac{1}{\sqrt{2015}}\right)+\left(\sqrt{2016}-\frac{1}{\sqrt{2016}}\right)>\sqrt{2015}+\sqrt{2016}\)
=>\(\frac{2016}{\sqrt{2015}}+\frac{2015}{\sqrt{2016}}>\sqrt{2015}+\sqrt{2016}\)
\(\sqrt{2016}+\sqrt{2015}>\sqrt{2015}+\sqrt{2014}\)
=> \(\frac{1}{\sqrt{2016}+\sqrt{2015}}