Sunday, October 13, 2019

proof verification - Proving inequality $ 1+frac14+frac19+cdots+frac1{n^2}le 2-frac1n$ using induction


Question:




Prove $$\ 1+\frac{1}{4}+\frac{1}{9}+\cdots+\frac{1}{n^2}\le 2-\frac{1}{n}, \text{ for all natural } n$$



My attempt:


Base Case: $n=1$ is true:


I.H: Suppose $1+\frac{1}{4}+\frac{1}{9}+\cdots+\frac{1}{k^2}\le 2-\frac{1}{k},$ for some natural $k.$


Now we prove true for $n = k+1$


$$ 1+\frac{1}{4}+\cdots+\frac{1}{k^2}+\frac{1}{\left(k+1\right)^2}\le 2-\frac{1}{k}+\frac{1}{\left(k+1\right)^2},\text{ by induction hypothesis} $$


Now how do I show that $2-\frac{1}{k}+\frac{1}{\left(k+1\right)^2}\le 2-\frac{1}{\left(k+1\right)}\text{ ?}$


Have I done everything correctly up until here?



If yes, how do I show this inequality is true?


Any help would be appreciated.


Answer



You are right!


We need to prove that: $$\frac{1}{(k+1)^2}<\frac{1}{k}-\frac{1}{k+1}$$ or $$\frac{1}{(k+1)^2}<\frac{1}{k(k+1)},$$ which is obvious.


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