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Let f, g : R → R be defined, respectively by f(x) = x + 1 and g(x) = 2x – 3. Find f + g, f – g and f/g. Find the domain in each case. |
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Answer» Given, f(x) = x + 1 and g(x) = 2x – 3 Clearly, Both f(x) and g(x) exist for all real values of x. Hence, Domain of f = Domain of g = R Range of f = Range of g = R i. f + g We know, (f + g)(x) = f(x) + g(x) ⇒ (f + g)(x) = x + 1 + 2x – 3 ∴ (f + g)(x) = 3x – 2 Domain of f + g = Domain of f ∩ Domain of g ⇒ Domain of f + g = R ∩ R ∴ Domain of f + g = R Thus, f + g : R → R is given by (f + g)(x) = 3x – 2 ii. f – g We know (f – g)(x) = f(x) – g(x) ⇒ (f – g)(x) = x + 1 – (2x – 3) ⇒ (f – g)(x) = x + 1 – 2x + 3 ∴ (f – g)(x) = –x + 4 Domain of f – g = Domain of f ∩ Domain of g ⇒ Domain of f – g = R ∩ R ∴ Domain of f – g = R Thus, f – g : R → R is given by (f – g)(x) = –x + 4 iii. \(\frac{f}{g}\) We know, (\(\frac{f}{g}\))(x) = \(\frac{f(x)}{g(x)}\) ∴ (\(\frac{f}{g}\))(x) = \(\frac{x+1}{2x-3}\) Clearly, (\(\frac{f}{g}\))(x) is defined for all real values of x, Except for the case when 2x – 3 = 0 or x = \(\frac{3}{2}\). When x = \(\frac{3}{2}\),(\(\frac{f}{g}\))(x) will be undefined as the division result will be indeterminate. Thus, Domain of \(\frac{f}{g}\) = R – \(\{\frac{3}{2}\}\) Thus, \(\frac{f}{g}\) : R – \(\{\frac{3}{2}\}\) → R is given by (\(\frac{f}{g}\))(x) = \(\frac{x+1}{2x-3}\) |
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