Question: Failure to convert from ln to arccosh

Hi all.

I was trying to perform a simple integration and convert the answer to a inverse hyperbolic function as follows:

int(9/sqrt(81*x^2-4),x);

simplify(%);

convert(%,arctrigh);

But the command convert fails to do the conversion. I should obtain arcosh(9/2*x)+c.   But the answer is in terms of the natural logarithm: ln(9*x+sqrt(81*x^2-4))

How do I manipulate the answer so that I can get the inverse hyperbolic answer?

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