This question is actually from [_MSE_](https://math.stackexchange.com/questions/2921750/infinite-sum-of-reciprocals-of-squares-of-lengths-of-tangents-from-origin-to-the). I had to post it here due to the lack of response there even after placing a bounty. Here goes the question 

> Let tangents be drawn to the curve $y=\sin x$ from the origin. Let the points of contact of these tangents with the curve be $(x_k,y_k)$ where $x_k\gt 0 ;(k\ge 1)$ such that $x_k\in (\pi k,  (k+1)\pi)$ and $$a_k=\sqrt {x_k^2+y_k^2}$$ (Which is basically the distance between the corresponding point of contact and the origin i.e. the length of tangent from origin)  . 

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I wanted to know the value of  
>$$\sum_{k=1}^{\infty} \frac {1}{a_k ^2}$$


Now this question has just popped out in my brain and is not copied from any assignment or any book so I don't know whether it will finally reach a conclusion or not. 

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I tried writing the equation of tangent to this curve from origin and then finding the points of contact but did not get a proper result which just that the $x$ coordinates of the points of contact will be the positive solutions of the equation $\tan x=x$ 

On searching internet for sometime about the solutions of $\tan x=x$ I got two important properties of this equation.  If $(\lambda _n)_{n\in N}$ denote the roots of this equation then 
>$$1)\sum_n^{\infty} \lambda _n \to \infty$$ $$2)\sum_n^{\infty} \frac {1}{\lambda _n^2} =\frac {1}{10}$$

But were not of much help. 

I also tried writing the points in polar coordinates to see if that could be of some help but I still failed miserably.

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On trying a bit more using some coordinate geometry I found that the locus of the points of contact is $$x^2-y^2=x^2y^2$$

Hence for third sum we just need to find $$\sum_{k=1}^{\infty} \frac {\lambda _k ^2 +1}{\lambda _k ^2 (\lambda _k ^2 +2)}=\sum_{k=1}^{\infty} \frac {1}{\lambda _k ^2} -\sum_{k=1}^{\infty} \frac {1}{\lambda _k ^2 (\lambda _k ^2 +2)}=\frac {1}{10} -\sum_{k=1}^{\infty} \frac {1}{\lambda _k ^2 (\lambda _k ^2 +2)}=\frac {1}{10} -\sum_{k=1}^{\infty} \frac {1}{2\lambda _k ^2} +\sum_{k=1}^{\infty} \frac {1}{2(\lambda _k ^2 +2)} =\frac {1}{20}+\frac {1}{2}\sum_{k=1}^{\infty} \frac {1}{\lambda _k ^2 +2} 
 $$

Now for the second summation I did think about it to form a series but for the roots to be $\lambda _k^2 +2$ we just need to substitute $x\to \sqrt {x−2}$ in power series of $\frac {\sin x-x\cos x}{x^3}$ and then get the result but it was still a lot confusing for me. 

Using $x\to\sqrt {x-2}$ in the above power series and using  [_Wolfy_](http://m.wolframalpha.com/input/?i=power+series+of+%28sin+%28√%28x-2%29%29-+%28√%28x-2%29%29*cos+%28√%28x-2%29%29%29%2F%28√%28%28x-2%29³%29%29) I have got a series. So we need ratio of coefficient of $x$ to the constant term so is the value of second summation equal to
$$\frac {5\sqrt 2\sinh(\sqrt 2)−6\cosh(\sqrt 2)}{4(2\cosh(\sqrt 2)−\sqrt 2\sinh(\sqrt 2))}?$$

Is this value correct or did I do it wrong? 

I would also like to know if there is some other method to solve this problem