Let A = {–2, –1, 0, 1, 2} and f : A → Z be a function defined by f(x) = x2 – 2x – 3. Find:
i. range of f i.e. f(A)
ii. pre-images of 6, –3 and 5
Given A = {–2, –1, 0, 1, 2}
f : A → Z such that f(x) = x2 – 2x – 3
i. range of f i.e. f(A)
A is the domain of the function f. Hence, range is the set of elements f(x) for all x ∈ A.
Substituting x = –2 in f(x), we get
f(–2) = (–2)2 – 2(–2) – 3
⇒ f(–2) = 4 + 4 – 3
∴ f(–2) = 5
Substituting x = –1 in f(x), we get
f(–1) = (–1)2 – 2(–1) – 3
⇒ f(–1) = 1 + 2 – 3
∴ f(–1) = 0
Substituting x = 0 in f(x), we get
f(0) = (0)2 – 2(0) – 3
⇒ f(0) = 0 – 0 – 3
∴ f(0) = –3
Substituting x = 1 in f(x), we get
f(1) = 12 – 2(1) – 3
⇒ f(1) = 1 – 2 – 3
∴ f(1) = –4
Substituting x = 2 in f(x), we get
f(2) = 22 – 2(2) – 3
⇒ f(2) = 4 – 4 – 3
∴ f(2) = –3
Thus, the range of f is {5, 0, –3, –4}.
ii. pre-images of 6, –3 and 5
Let x be the pre-image of 6 ⇒ f(x) = 6
⇒ x2 – 2x – 3 = 6
⇒ x2 – 2x – 9 = 0
However,
Thus, there exists no pre-image of 6.
Now, let x be the pre-image of –3 ⇒ f(x) = –3
⇒ x2 – 2x – 3 = –3
⇒ x2 – 2x = 0
⇒ x(x – 2) = 0
∴ x = 0 or 2
Clearly, both 0 and 2 are elements of A.
Thus, 0 and 2 are the pre-images of –3.
Now, let x be the pre-image of 5 ⇒ f(x) = 5
⇒ x2 – 2x – 3 = 5
⇒ x2 – 2x – 8= 0
⇒ x2 – 4x + 2x – 8= 0
⇒ x(x – 4) + 2(x – 4) = 0
⇒ (x + 2)(x – 4) = 0
∴ x = –2 or 4
However, 4 ∉ A but –2 ∈ A
Thus, –2 is the pre-images of 5.