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anemone
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Let $a\in \Bbb{Z^+}$, prove that $\dfrac{2}{2-\sqrt{2}}>\dfrac{1}{1\sqrt{1}}+\dfrac{1}{2\sqrt{2}}+\dfrac{1}{3\sqrt{3}}+\cdots+\dfrac{1}{a\sqrt{a}}$.
MarkFL said:My solution:
Let:
\(\displaystyle S\equiv\sum_{k=1}^{\infty}\left(k^{-\frac{3}{2}}\right)\)
Using graphical tools, we can easily see that:
\(\displaystyle S<1+\int_{1}^{\infty}u^{-\frac{3}{2}}\,du\)
To evaluate the integral $I$, we may write:
\(\displaystyle I=\lim_{t\to\infty}\left(\int_{1}^{t}u^{-\frac{3}{2}}\,du\right)\)
Applying the anti-derivative form of the FTOC, there results:
\(\displaystyle I=-2\lim_{t\to\infty}\left(\left.u^{-\frac{1}{2}}\right|_{1}^t\right)=2\)
Hence, we have $S<3$ and we may state
\(\displaystyle S_a<S<3<\frac{2}{2-\sqrt{2}}=2+\sqrt{2}\)
Shown as desired. :)
my solution:anemone said:Let $a\in \Bbb{Z^+}$, prove that $\dfrac{2}{2-\sqrt{2}}>\dfrac{1}{1\sqrt{1}}+\dfrac{1}{2\sqrt{2}}+\dfrac{1}{3\sqrt{3}}+\cdots+\dfrac{1}{a\sqrt{a}}$.
Albert said:my solution:
$A=\dfrac{2}{2-\sqrt{2}}=\dfrac {1}{1-\dfrac{\sqrt 2}{2}}$
$B=\dfrac{1}{1\sqrt{1}}+\dfrac{1}{2\sqrt{2}}+\dfrac{1}{3\sqrt{3}}+\cdots+\dfrac{1}{a\sqrt{a}}$
$ =1+\dfrac {\sqrt 2}{2^2}+\dfrac {\sqrt 3}{3^2}+\dfrac {\sqrt 4}{4^2}+------+\dfrac {\sqrt a}{a^2}$
$A$ is a infinite geometric sum with ratio $r=\dfrac {\sqrt 2}{2}$, and first term $A_1=1$
$\therefore A>B$
The "Prove Inequality Challenge for $a\in \Bbb{Z^+}$" is a mathematical challenge that asks participants to prove a given inequality for all positive integers, denoted by $\Bbb{Z^+}$. This challenge is often used in mathematical competitions and exercises to test a person's understanding of mathematical concepts and their ability to prove statements using mathematical reasoning.
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Yes, there are many resources available to help with the "Prove Inequality Challenge". These include textbooks, online tutorials, and practice problems and solutions. It is also helpful to work with a study group or seek guidance from a teacher or mentor who can provide feedback and assistance with solving challenging inequalities.
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