# can´t solve this kind of exponential equations

I have no clue how can i solve this kind of exponential equation in closed form:

$a^n - b^n \le c$

where $a > 1$ and $-1 < b < 0$

thank you very much for your help

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Um, what are you trying to solve here? Find a bound for $c$? –  Johannes Kloos Nov 20 '12 at 10:56
In what closed form, what do you aim to achieve? –  martini Nov 20 '12 at 10:57
actually I want to find a bound for $n$ –  Michele Fratello Nov 26 '12 at 12:17