With these sorts of functions the value of x that makes the bottom part of the fraction equal zero is a vertical asymptote .
So in this example f(x)=x+12x−1
2x−1=0
2x=1
x=12
Therefore f(x) is defined for every value of x except for x=12
Therefore the domain is x∈[−∞,12 )∪ (12,∞]
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