<s>[INST] Derivative of $\frac{d}{dx}\ln (x+\sqrt{x^2+1})$ Show that $\frac{d}{dx}\ln (x+\sqrt{x^2+1}) = \frac{1}{\sqrt{x^2+1}}$
So I've done this so far:
$$\frac{d}{dx}\ln (x+\sqrt{x^2+1}) = \frac{1+x(x^2+1)^{-0.5}}{x+(x^2+1)^{0.5}}$$
I have tried a few combinations but cannot attain the desired result. Could someone offer just a hint to get me going?
Thanks
 [/INST] Your differentiation is correct, and you applied the chain rule correctly. But as you note, your result can be simplified. 
Let's rewrite the fraction, expressing the term $x(x^2 + 1)^{-0.5}$ in the numerator as $\frac{x}{\sqrt {x^2 + 1}}$, then simply multiply the numerator and denominator by $\sqrt{x^2 + 1}$:
$$\begin{align}\frac{1+x(x^2+1)^{-0.5}}{x+(x^2+1)^{0.5}} & = \frac{1+\frac{x}{\sqrt{x^2+1}}}{x+\sqrt{x^2+1}}\\ \\
& = \frac{\sqrt{x^2 +1} + x}{{(x+\sqrt{x^2+1})\sqrt{x^2+1}}}\\ \\
& =\frac{\color{blue}{\bf x+\sqrt{x^2+1}}}{(\color{blue}{\bf x+\sqrt{x^2+1}})\sqrt{x^2+1}} \\ \\
& =\frac1{\sqrt{x^2+1}}\end{align}$$
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