How can I easily derive this tricky integral?

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does anybody know how to derive this integral?

\int \frac{dx}_{\sqrt{x^2 + a^2}} = ln(x+ \sqrt{x^2 + a^2})
 
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Looks like you already took it, wrongly. Looks more to me like an arctan. Some things are worth memorizing. Not all, but some.
 
Dick said:
Looks like you already took it, wrongly. Looks more to me like an arctan. Some things are worth memorizing. Not all, but some.

corrections are made above.
 
The easy way is to use hyperbolic functions. Substitute x=a*sinh(t). Then figure out what the arcsinh function looks like by solving a quadratic. Otherwise a trig substitution like x=a*tan(t).
 

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