Can we describe an original and inverse equation with one function?

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Let us say we have two real values, 1 and x.

I want to determine the absolute value of the difference or their sum between 1 and x without specifying whether I am dealing with 1 - x or 1 + x

For example,

somefunction(1 - x) == somefunction(1 + x) == someotherfunction(x, y)

My original goal was to attempt to simplify the PV and FV equation for annuities from

P{[1-(1+r)^-tm]/r} and P{[(1+r)^tm-1]/r}

for my notes. I'm not sure if this is possible, and your input is greatly appreciated.

My apologies for poor formatting. I am not familiar with MathJaX or LaTEX