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This applies to products sold in US. If the listed price is tax-inclusive, many businesses will list the price as P = p + p*r, where p is the price before tax, and r the tax rate.
However if you do so, you have to pay taxes on p*r: the amount is (p*r)*r = p*(r^2). You can factor this into you final price, adding p*(r^2) so that P becomes P = p + p*r + p*(r^2). But then you must pay taxes on p*(r^2) and so on recursively in an infinite loop.
Your final price will be
P = p*(1 + r + r^2 + r^3 + ...) = p / (1-r).
Wondering how many businesses have figured that out.