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Relations And Functions

Question
CBSEENMA12035706

If the function f: R rightwards arrow R  be given by straight f left parenthesis straight x right parenthesis space equals space straight x squared plus 2 space space and space straight g space colon thin space straight R rightwards arrow space straight R be given by straight g left parenthesis straight x right parenthesis space equals fraction numerator straight x over denominator straight x minus 1 end fraction comma space straight x not equal to 1 comma find fog and gof and hence find fog (2) and gof ( −3).

Solution

Given that straight f left parenthesis straight x right parenthesis space equals space straight x squared plus 2 space and space straight g left parenthesis straight x right parenthesis space equals space fraction numerator straight x over denominator straight x minus 1 end fraction
Let us find fog:
space fog space equals space straight f open parentheses straight g left parenthesis straight x right parenthesis close parentheses
rightwards double arrow space space fog space equals space open parentheses straight g left parenthesis straight x right parenthesis close parentheses squared plus 2
rightwards double arrow space fog space equals open parentheses fraction numerator straight x over denominator straight x minus 1 end fraction close parentheses squared plus 2
rightwards double arrow fog space equals fraction numerator straight x squared plus 2 left parenthesis straight x minus 1 right parenthesis squared over denominator left parenthesis straight x minus 1 right parenthesis squared end fraction
rightwards double arrow fog space equals fraction numerator straight x squared plus 2 left parenthesis straight x squared minus 2 straight x plus 1 right parenthesis over denominator straight x squared minus 2 straight x plus 1 end fraction
rightwards double arrow fog space equals fraction numerator 3 straight x squared minus 4 straight x plus 2 over denominator straight x squared minus 2 straight x plus 1 end fraction
Therefore, left parenthesis fog right parenthesis space left parenthesis 2 right parenthesis space equals space fraction numerator 3 cross times 2 squared minus 4 cross times 2 plus 2 over denominator 2 squared minus 2 cross times 2 plus 1 end fraction
rightwards double arrow left parenthesis fog right parenthesis thin space left parenthesis 2 right parenthesis space equals space fraction numerator 12 minus 8 plus 2 over denominator 4 minus 4 plus 1 end fraction space equals 6
Now space let space us space find space gof colon
gof space equals space straight g open parentheses straight f left parenthesis straight x right parenthesis close parentheses
rightwards double arrow space space gof space equals space fraction numerator straight f left parenthesis straight x right parenthesis over denominator straight f left parenthesis straight x right parenthesis minus 1 end fraction
rightwards double arrow gof space equals space fraction numerator straight x squared plus 2 over denominator straight x squared plus 2 minus 1 end fraction
rightwards double arrow space gof space equals fraction numerator straight x squared plus 2 over denominator straight x squared plus 1 end fraction
 Therefore comma space left parenthesis gof right parenthesis thin space left parenthesis negative 3 right parenthesis space equals space fraction numerator left parenthesis negative 3 right parenthesis squared plus 2 over denominator left parenthesis negative 3 right parenthesis squared plus 1 end fraction equals space fraction numerator 9 plus 2 over denominator 9 plus 1 end fraction equals 11 over 10

Some More Questions From Relations and Functions Chapter

Show that the relation R in the set {1, 2, 3} given by R = {(1, 1), (2, 2), (3, 3), (1, 2), (2, 3)} is reflexive but neither symmetric nor transitive.

Show that the relation R in the set {1, 2, 3} given by R = {(1, 2), (2, 1)} is symmetric but neither reflexive nor transitive.

Give an example of a relation which is

(i) Symmetric but neither reflexive nor transitive.
(ii) Transitive but neither reflexive nor symmetric.
(iii) Reflexive and symmetric but not transitive.
(iv) Reflexive and transitive but not symmetric.
(v) Symmetric and transitive but not reflexive.

Let L be the set of all lines in a plane and R be the relation in L defined as R = {(L1, L2) : L1 is perpendicular to L2}. Show that R is symmetric but neither reflexive nor transitive.

 Determine whether each of the following relations are reflexive, symmetric and transitive :

(i) Relation R in the set A = {1, 2, 3,....., 13, 14} defined as

R = {(x, y) : 3 x – y = 0}

(ii) Relation R in the set N of natural numbers defined as R = {(x, y) : y = x + 5 and x < 4} (iii) Relation R in the set A = {1, 2, 3, 4, 5, 6} as R = {(x,y) : y is divisible by x} (iv) Relation R in the set Z of all integers defined as R = {(x,y) : x – y is an integer}

(v) Relation R in the set A of human beings in a town at a particular time given by
(a)    R = {(x, y) : x and y work at the same place}
(b)    R = {(x,y) : x and y live in the same locality}
(c)    R = {(x, y) : x is exactly 7 cm taller than y}
(d)    R = {(x, y) : x is wife of y}
(e)    R = {(x,y) : x is father of y}

Check whether the relation R defined in the set {1, 2, 3, 4, 5, 6} as R = {(a, b) : b = a + 1} is reflexive, symmetric or transitive.

Show that the relation R in R defined as R = {(a, b) : a ≤ b}, is reflexive and transitive but not symmetric.

Check whether the relation R in R defined by R = {(a,b) : a ≤ b3} is refleive, symmetric or transitive.

If R and R’ arc reflexive relations on a set then so are R ∪ R’ and R ∩ R’.