Q. 44.6( 9 Votes )

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

Answer :

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.


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