Let f, g be two real functions defined by and
. Then, describe each of the following functions.
i. f + g
ii. g – f
iii. fg
iv.
v.
vi.
vii. f2 + 7f
viii.
Given and
We know the square of a real number is never negative.
Clearly, f(x) takes real values only when x + 1 ≥ 0
⇒ x ≥ –1
∴ x ∈ [–1, ∞)
Thus, domain of f = [–1, ∞)
Similarly, g(x) takes real values only when 9 – x2 ≥ 0
⇒ 9 ≥ x2
⇒ x2 ≤ 9
⇒ x2 – 9 ≤ 0
⇒ x2 – 32 ≤ 0
⇒ (x + 3)(x – 3) ≤ 0
⇒ x ≥ –3 and x ≤ 3
∴ x ∈ [–3, 3]
Thus, domain of g = [–3, 3]
i. f + g
We know (f + g)(x) = f(x) + g(x)
Domain of f + g = Domain of f ∩ Domain of g
⇒ Domain of f + g = [–1, ∞) ∩ [–3, 3]
∴ Domain of f + g = [–1, 3]
Thus, f + g : [–1, 3] → R is given by
ii. f – g
We know (f – g)(x) = f(x) – g(x)
Domain of f – g = Domain of f ∩ Domain of g
⇒ Domain of f – g = [–1, ∞) ∩ [–3, 3]
∴ Domain of f – g = [–1, 3]
Thus, f – g : [–1, 3] → R is given by
iii. fg
We know (fg)(x) = f(x)g(x)
As earlier, domain of fg = [–1, 3]
Thus, f – g : [–1, 3] → R is given by
iv.
We know
As earlier, domain of = [–1, 3]
However, is defined for all real values of x ∈ [–1, 3], except for the case when 9 – x2 = 0 or x = ±3
When x = ±3, will be undefined as the division result will be indeterminate.
⇒ Domain of = [–1, 3] – {–3, 3}
∴ Domain of = [–1, 3)
Thus, : [–1, 3) → R is given by
v.
We know
As earlier, domain of = [–1, 3]
However, is defined for all real values of x ∈ [–1, 3], except for the case when x + 1 = 0 or x = –1
When x = –1, will be undefined as the division result will be indeterminate.
⇒ Domain of = [–1, 3] – {–1}
∴ Domain of = (–1, 3]
Thus, : (–1, 3] → R is given by
vi.
We know (f – g)(x) = f(x) – g(x) and (cf)(x) = cf(x)
As earlier, Domain of = [–1, 3]
Thus, : [–1, 3] → R is given by
vii. f2 + 7f
We know (f2 + 7f)(x) = f2(x) + (7f)(x)
⇒ (f2 + 7f)(x) = f(x)f(x) + 7f(x)
Domain of f2 + 7f is same as domain of f.
∴ Domain of f2 + 7f = [–1, ∞)
Thus, f2 + 7f : [–1, ∞) → R is given by
viii.
We know and (cg)(x) = cg(x)
Domain of = Domain of g = [–3, 3]
However, is defined for all real values of x ∈ [–3, 3], except for the case when 9 – x2 = 0 or x = ±3
When x = ±3, will be undefined as the division result will be indeterminate.
⇒ Domain of = [–3, 3] – {–3, 3}
∴ Domain of = (–3, 3)
Thus, : (–3, 3) → R is given by