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The domain of a function is set of all numbers, which can be put to formula of a function. The domain can be obtain from graph of a function too. Denoted by D, D_f, X. Examples. y = root of x - D = [0,∞) - a set of a positive numbers and zero, because not allowed a root of a negative number. y = 1/x - D = R\{0} - set of reals without zero, because not allowed divide by 0. 1/x = 1:x.