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Definition of domain : Set all values of 'x' for which f(x) is defined
f(x) is defined at x= a means f(a) may be positive or negative or zero.
Example f(x) = 1/x f(2) = 1/2 = 0.5 implies f(x) is defined at x = 2
f(0) = 1/0 = no value implies f(x) is not defined at x = 0.