If $xy + yz + zx = 1$, then show that 

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> $$\dfrac{x}{1-x^2} + \dfrac{y}{1-y^2} + \dfrac{z}{1-z^2} = \dfrac{4xyz}{(1-x^2)(1-y^2)(1-z^2)}$$
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I tried doing the sum algebraically, that is, by solving the LHS but, that method isn't really getting us anywhere.

Also, I found this sum in an a book on trigonometry. So how can I solve it with trigonometry?