## CBSE Class 12 Mathematics Important Questions Chapter 1 Relations and Functions

## 1 Mark Questions

1. A Relation R:AàA is said to be Reflexive if ——— for every a A where A is non

empty set.

Ans: (a, a) R

2. A Relation R:AàA is said to be Symmetric if ———- a,b, A

Ans: (a, b) R, (b, a) R

3. A Relation R:AàA is said to be Transitive if ————- a,b,c A

Ans: (a, b)R, and (b, c)R (a, c) R.

4. Define universal relation? Give example.

Ans: A Relation R in a set A called universal relation if each element of A is related to every element of A. Ex. Let = {2,3,4}

R = (AA) = {(2,2),(2,3) (2,4) (3,2) (3,3) (3,4) (4,2) (4,3) (4,4) }

5. What is trivial relation?

Ans: Both the empty relation and the universal relation are some time called trivial relation.

6. Prove that the function f: R à R, given by f(x) = 2x, is one – one.

Ans: f is one – one as f(x1) = f (x1)

2x1 = 2x2

x1 = x2

Prove.

7. State whether the function is one – one, onto or bijective f: R à R defined by f(x) = 1+ x2

Ans: Let x1, x2 x

If f(x1) = f(x2)

Hence not one – one

8. Let S = {1, 2, 3}

Determine whether the function f: S à S defined as below have inverse.

f = {(1, 2), (2, 1), (3, 1)}

Ans: f(2) = 1 f(3) = 1,

f is not one – one, So that that f is not invertible.

9. Find gof f(x) = |x|, g(x) = |5x + 1|

Ans: gof (x) = g [f(x)]

= g [(x)]

=

10. Let f, g and h be function from R to R show that (f + g) oh = foh + goh

Ans: L.H.S = (f + g) oh

= {(f + g) oh} (x)

= (f + g) h (x)

= f [h (x)] + g [h (x)]

= foh + goh

9. If a * b = a + 3b2, then find 2 * 4

Ans: 2 * 4 = 2 + 3 (4)2

= 2 + 3 16

= 2 + 48

= 50

11. Show that function f: N à N, given by f(x) = 2x, is one – one.

Ans: the function f is one – one, for

f(x1) = f(x2)

2x1 = 2x2

x1 = x2

12. State whether the function is one – one, onto or bijective f: R à R defined by f(x) = 3 – 4x

Ans: is x1, x2 R

f(x1) = f(x2)

3 – 4x1 = 3 – 4x2

x1 = x2

Hence one – one

Y = 3 – 4x

= y

Hence onto also.

13. Let S = {1, 2, 3}

Determine whether the function f: S à S defined as below have inverse.

f = {(1, 1), (2, 2), (3, 3)}

Ans: f is one – one and onto, so that f is invertible with inverse f-1 = {(1, 1) (2, 2) (3, 3)}

14. Find got f(x) = |x|, g(x) = |5x -2|

Ans: fog (x) = f(g x)

= f{|5x – 2|)

= |5x – 2|

15. Consider f: {1, 2, 3} à {a, b, c} given by f(1) = a, f(2) = b and f(3) = c find f-1 and show

that (f-1)-1 = f

Ans: f = {(1, a) (2, b) (3, c)}

f-1 = { (a, 1) (b, 2) (c, 3)}

(f -1) -1 = {(1, a) (2, b) (3, c)}

Hence (f-1)-1 = f.

16. If f(x) = x + 7 and g(x) = x – 7, x R find (fog) (7)

Ans: (fog) (x) = f[g(x)]

= f(x – 7)

= x – 7 + 7

= x

(fog) (7) = (7)

17. What is a bijective function?

Ans: A function f: X à Y is said to be one – one and onto (bijective), if f is both one – one and onto.

18 Let f: R à R be define as f(x) = x4 check whether the given function is one – one onto,

or other.

Ans: Let x1, x2 R

If f(x1) = f(x2)

Not one – one

Not onto.

19 Let S = {1, 2, 3}

Determine whether the function f: S à S defined as below have inverse.

f = {(1, 3) (3, 2) (2, 1)}

Ans: f is one – one and onto, Ao that f is invertible with f-1 = {(3,1) (2, 3) (1, 2)}

20 Find gof where f(x) = 8x3, g(x) = x1/3

Ans: gof (x) = g[f(x)]

= g (8x3)

=

= 2x

21. Let f, g and h be function from R + R. Show that (f.g) oh = (foh). (goh)

Ans: (f. g) oh

(f. g) h (x)

f[h(x)]. g[h(x)]

foh. goh

21. Let * be a binary operation defined by a * b = 2a + b – 3. find 3 * 4

Ans: 3 * 4 = 2 (3) + 4-3 = 7

22. show that a one – one function f: {1, 2, 3} à {1, 2, 3} must be onto.

Ans: Since f is one – one three element of {1, 2, 3} must be taken to 3 different element of the co – domain {1, 2, 3} under f. hence f has to be onto.

23. f: R à R be defined as f(x) = 3x check whether the function is one – one onto or other

Ans: Let

24. Let S = {1, 2, 3}

Determine whether the function f: S à S defined as below have inverse.

f = { (1, 2) (2, 1) (3, 1) }

Ans:f(2) = 1, f(3) =1

f is not one – one so that f is not invertible

Hence no inverse

25. Find fog f(x) = 8x3, g(x) = x1/3

Ans: fog (x) = f(gx)

= 8x

26. If f: R à R be given by f(x) = , find fof (x)

Ans:

=

= x

27. If f(x) is an invertible function, find the inverse of f(x) =

Ans: Let f(x) = y

## 4 Marks Questions

1. Let T be the set of all triangles in a plane with R a relation in T given by

R = {(T1, T2): T1 is congruent to T2}.

Show that R is an equivalence relation.

Ans. R is reflexive, since every is congruent to itself.

(T1T2)R similarly (T2T1) R

since T1 T2

(T1T2) R, and (T2,T3) R

(T1T3)R Since three triangles are

congruent to each other.

2. Show that the relation R in the set Z of integers given byR={(a, b) : 2 divides a-b}. is equivalence relation.

Ans. R is reflexive , as 2 divide a-a = 0

((a,b)R ,(a-b) is divide by 2

(b-a) is divide by 2 Hence (b,a) R hence symmetric.

Let a,b,c Z

If (a,b) R

And (b,c) R

Then a-b and b-c is divided by 2

a-b +b-c is even

(a-c is even

(a,c) R

Hence it is transitive.

3. Let L be the set of all lines in plane and R be the relation in L define if R = {(l1, L2 ): L1 is to L2 } . Show that R is symmetric but neither reflexive nor transitive.

Ans. R is not reflexive , as a line L1 cannot be to itself i.e (L1,L1 ) R

L1 L2

L2 L1

(L2,L1)R

L1 L2 and L2 L3

Then L1 can never be to L3 in fact L1 || L3

i.e (L1,L2) R, (L2,L3) R.

But (L1, L3) R

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

Ans. R = {(a,b): b= a+1}

Symmetric or transitive

R = {(1,2) (2,3) (3,4) (4,5) (5,6) }

R is not reflective , because (1,1) R

R is not symmetric because (1,2) R but (2,1) R

(1,2) R and (2,3) R

But (1,3) R Hence it is not transitive

5. Let L be the set of all lines in xy plane and R be the relation in L define as R = {(L1, L2): L1 || L2} Show then R is on equivalence relation.

Find the set of all lines related to the line y=2x+4.

Ans. L1||L1 i.e (L1, L1) R Hence reflexive

L1||L2 then L2 ||L1 i.e (L1L2) R

(L2,L) R Hence symmetric

We know the

L1||L2 and L2||L3

Then L1|| L3

Hence Transitive . y = 2x+K

When K is real number.

6. Show that the relation in the set R of real no. defined R = {(a, b) : a b3 }, is neither reflexive nor symmetric nor transitive.

Ans. (i) (a, a) Which is false R is not reflexive.

(ii) Which is false R is not symmetric.

(iii) Which is false

7. Let A = N N and * be the binary operation on A define by (a, b) * (c, d) = (a + c, b + d)Show that * is commutative and associative.

Ans. (i) (a, b) * (c, d) = (a + c, b + d)

= (c + a, d + b)

= (c, d) * (a, b)

Hence commutative

(ii) (a, b) * (c, d) * (e, f)

= (a + c, b + d) * (e, f)

= (a + c + e, b + d + f)

= (a, b) * (c + e, d + f)

= (a, b) * (c, d) * (e, f)

Hence associative.

8. Show that if f: is defining by f(x) = and g: is define by

g(x) = then fog = IA and gof = IB when ; IA (x) = x, for all x A, IB(x) = x, for all x B are called identify function on set A and B respectively.

Ans. gof (x) =

Which implies that gof = IB

And Fog = IA

9. Let f: N à N be defined by f(x) =

Examine whether the function f is onto, one – one or bijective

Ans.

f is not one – one

1 has two pre images 1 and 2

Hence f is onto

f is not one – one but onto.

10. Show that the relation R in the set of all books in a library of a collage given by R ={(x, y) : x and y have same no of pages}, is an equivalence relation.

Ans. (i) (x, x) R, as x and x have the same no of pages for all xR R is reflexive.

(ii) (x, y) R

x and y have the same no. of pages

y and x have the same no. of pages

(y, x) R

(x, y) = (y, x) R is symmetric.

(iii) if (x, y) R, (y, y) R

(x, z) R

R is transitive.

11. Let * be a binary operation. Given by a * b = a – b + abIs * :

(a) Commutative

(B) Associative

Ans. (i) a * b = a – b + ab

b * a = b – a + ab

a * b b * a

(ii) a * (b * c) = a * (b – c + bc)

= a – (b – c + bc) + a. (b – c + bc)

= a – b + c – bc + ab – ac + abc

(a * b) * c = (a – b + ab) * c

= [ (a – b + ab) – c ] + ( a – b + ab)

= a- b + ab – c + ac – bc + abc

a * (b * c) (a * b) * c.

12. Let f: R à R be f (x) = 2x + 1 and g: R à R be g(x) = x2 – 2 find (i) gof (ii) fog

Ans. (i) gof (x) = g[f(x)]

= g (2x + 1)

= (2x + 1)2 – 2

(ii) fog (x) = f (fx)

= f (2x + 1)

= 2(2x + 1) + 1

= 4x + 2 + 1 = 4x + 3

13. Let A = R – {3} and B = R- {1}. Consider the function of f: A à B defined by

f(x) = is f one – one and onto.

Ans. Let x1 x2 A

Such that f(x1) = f(x2)

f is one – one

Hence onto

14. Show that the relation R defined in the set A of all triangles asR = { is similar to T2 }, is an equivalence relation. Consider three right angle triangles T1 with sides 3, 4, 5. T2 with

sides 5, 12, 13 and T3 with sides 6, 8, 10. Which triangles among T1, T2 and T3 are related?

Ans. (i) Each triangle is similar to at well and thus (T1, T1) R

R is reflexive.

(ii) (T1, T2) R

T1 is similar to T2

T2 is similar to T1

(T2, T1) R

R is symmetric

(iii) T1 is similar to T2 and T2 is similar to T3

T1 is similar to T3

(T1, T3) R

R is transitive.

Hence R is equivalence

(II) part T1 = 3, 4, 5

T2 = 5, 12, 13

T3 = 6, 8, 10

T1 is relative to T3.

15. Determine which of the following operation on the set N are associative and which are commutative.

(a) a * b = 1 for all a, b N

(B) a * b = for all a, b, N

Ans. (a) a * b = 1

b * a = 1

for all a, b N also

(a * b) * c = 1 * c = 1

a * (b * c) = a * (1) = 1 for all, a, b, c R N

Hence R is both associative and commutative

(b) a * b = , b * a =

Hence commutative.

(a * b) * c =

=

=

* is not associative.

17. Let A and B be two sets. Show that f: A B à B A such that f(a, b) = (b, a) is a bijective function.

Ans. Let (a1 b1) and (a2, b2) A B

(i) f(a1 b1) = f(a2, b2)

b1 = b2 and a1 = a2

(a1 b1) = (a2, b2)

Then f(a1 b1) = f(a2, b2)

(a1 b1) = (a2, b2) for all

(a1 b1) = (a2, b2) A B

(ii) f is injective,

Let (b, a) be an arbitrary

Element of B A. then b B and a A

(a, b) ) (A B)

Thus for all (b, a) B A their exists (a, b) ) (A B)

Hence that

f(a, b) = (b, a)

So f: A B à B A

Is an onto function.

Hence bijective

18. Show that the relation R defined by (a, b) R (c, d) a + b = b + c on the set N N is an equivalence relation.

Ans. (a, b) R (c, d) a + b = b + c where a, b, c, d N

(a, b ) R (a, b) a + b = b + a (a, b) N N

R is reflexive

(a, b) R (c, d) a + b

= b + c (a, b ) (c, d) N N

d + a = c + b

c + b = d + a

(c, d) R (a, b) (a, b), (c, d) N N

Hence reflexive.

(a, b) R (c, d) a + d = b + c (1) (a, b), (c, d) N N

(c, d) R (e, f) c + f = d + e (2) (c, d), (e, f) N N

Adding (1) and (2)

(a + b) + [(+f)] = (b + c) + (d + e)

a + f = b + e

(a, b) R (e, f)

Hence transitive

So equivalence

19. Let * be the binary operation on H given by a * b = L. C. M of a and b. find

(a) 20 * 16

(b) Is * commutative

(c) Is * associative

(d) Find the identity of * in N.

Ans. (i) 20 * 16 = L. C.M of 20 and 16

= 80

(ii) a * b = L.C.M of a and b

= L.C.M of b and a

= b * a

(iii) a * (b * c) = a * (L.C.M of b and c)

= L.C.M of (a and L.C.M of b and c)

= L.C.M of a, b and c

Similarity

(a * b) * c = L. C.M of a, b, and c

(iv) a * 1 = L.C.M of a and 1= a

=1

20. If the function f: R à R is given by f(x) = and g: R à R is given by g(x) = 2x – 3, Find

(i) fog

(ii) gof. Is f-1 = g

(iii) fog = gof = x

Ans. (i) fog (x) = f [g(x)]

= f (2x – 3)

=

= x

(ii) gof (x) = g [f(x)]

= x

(iii) fog = gof = x

Yes,

21. Let L be the set of all lines in Xy plane and R be the relation in L define as R = {(L1, L2): L1 || L2} Show then R is on equivalence relation.

Find the set of all lines related to the line y=2x+4.

Ans. L1||L1 i.e (L1, L1) R Hence reflexive

L1||L2 then L2 ||L1 i.e (L1L2) R

(L2, L) R Hence symmetric

We know the

L1||L2 and L2||L3

Then L1|| L3

Hence Transitive. y = 2x+K

When K is real no.

### Relations and Functions Class 12 Maths MCQs

1. Let R be a relation on the set L of lines defined by l_{1} R l_{2} if l_{1} is perpendicular to l_{2}, then relation R is

(a) reflexive and symmetric

(b) symmetric and transitive

(c) equivalence relation

(d) symmetric

**Answer/Explanation**

Answer: d

Explaination: (d), not reflexive, as l_{1} R l_{2}

⇒ l_{1} ⊥ l_{1} Not true

Symmetric, true as l_{1} R l_{2} ⇒ l2R h

Transitive, false as l_{1} R l_{2}, l_{2} R l_{3}

⇒ l_{1} || l_{3} . l_{1} R l_{2}.

2. Given triangles with sides T_{1} : 3, 4, 5; T_{2} : 5, 12, 13; T_{3} : 6, 8, 10; T_{4} : 4, 7, 9 and a relation R in set of triangles defined as R = {(Δ_{1}, Δ_{2}) : Δ_{1} is similar to Δ_{2}}. Which triangles belong to the same equivalence class?

(a) T_{1} and T_{2}

(b) T_{2} and T_{3}

(c) T_{1} and T_{3}

(d) T_{1} and T_{4}

**Answer/Explanation**

Answer: c

Explaination: (c), T_{1} and T_{3} are similar as their sides are proportional.

3. Given set A ={1, 2, 3} and a relation R = {(1, 2), (2, 1)}, the relation R will be

(a) reflexive if (1, 1) is added

(b) symmetric if (2, 3) is added

(c) transitive if (1, 1) is added

(d) symmetric if (3, 2) is added

**Answer/Explanation**

Answer: c

Explaination: (c), here (1,2) e R, (2,1) € R, if transitive (1,1) should belong to R.

4. Given set A = {a, b, c). An identity relation in set A is

(a) R = {(a, b), (a, c)}

(b) R = {(a, a), (b, b), (c, c)}

(c) R = {(a, a), (b, b), (c, c), (a, c)}

(d) R= {(c, a), (b, a), (a, a)}

**Answer/Explanation**

Answer: b

Explaination: (b), A relation R is an identity relation in set A if for all a ∈ A, (a, a) ∈ R.

5. A relation S in the set of real numbers is defined as xSy ⇒ x – y+ √3 is an irrational number, then relation S is

(a) reflexive

(b) reflexive and symmetric

(c) transitive

(d) symmetric and transitive

**Answer/Explanation**

Answer: a

Explaination:

6. Set A has 3 elements and the set B has 4 elements. Then the number of injective functions that can be defined from set A to set B is

(a) 144

(b) 12

(c) 24

(d) 64

**Answer/Explanation**

Answer: c

Explaination: (c), total injective mappings/functions

= ^{4} P_{3} = 4! = 24.

7. Given a function lf as f(x) = 5x + 4, x ∈ R. If g : R → R is inverse of function ‘f then

(a) g(x) = 4x + 5

(b) g(x) = \(\frac{5}{4 x-5}\)

(c) g(x) = \(\frac{x-4}{5}\)

(d) g(x) = 5x – 4

**Answer/Explanation**

Answer: c

Explaination:

8. Let Z be the set of integers and R be a relation defined in Z such that aRb if (a – b) is divisible by 5. Then R partitions the set Z into ______ pairwise disjoint subsets.

**Answer/Explanation**

Answer:

Explaination: Five, as remainder can be 0, 1, 2, 3, 4.

9. Consider set A = {1, 2, 3 } and the relation R= {(1, 2)}, then? is a transitive relation. State true or false.

**Answer/Explanation**

Answer:

Explaination: True, as there is no situation

(a, b) ∈ R, (b, c) ∈ R Hence, transitive. We can also say, a relation containing only one element is transitive.

10. Every relation which is symmetric and transitive is reflexive also. State true or false.

**Answer/Explanation**

Answer:

Explaination: False,e.g.if R is arelationinset A = {2,3,4} defined as {(2, 3), (3, 2), (2, 2)} is symmetric and transitive but not reflexive.

11. Let R be a relation in set N, given by R = {(a, b): a = b – 2, b > 6} then (3, 8) ∈ R. State true or false with reason.

**Answer/Explanation**

Answer:

Explaination: False, as in (3, 8), b = 8

⇒ a = 8 – 2

⇒ a = 6, but here a = 3.

12. Let R be a relation defined as R = {(x, x), (y, y), (z, z), (x, z)} in set A = {x, y, z} then R is (reflexive/symmetric) relation.

**Answer/Explanation**

Answer:

Explaination: Reflexive, as for all a ∈ A, (a, a) ∈ R.

13. Let R be a relation in the set of natural numbers N defined by R = {(a, b) ∈ N × N: a < b}. Is relation R reflexive? Give a reason.

**Answer/Explanation**

Answer:

Explaination:

Given R = {(a, b) ∈ N × N: a < b}.

Not reflexive, as for (a, a) × R

⇒ a< a, not true.

14. Let A be any non-empty set and P(A) be the power set of A. A relation R defined on P(A) by X R Y ⇔ X ∩ Y = X, X, Y ∈ P(A). Examine whether ? is symmetric.

**Answer/Explanation**

Answer:

Explaination: X R Y ⇔ X ∩ Y = X ⇒ Y ∩ X = X ⇒ Y R X.

Hence, symmetric.

15. State the reason for the relation R in the set {1, 2, 3} given by R = {(1, 2), (2, 1)} not to be transitive. [NCERT; Delhi 2011]

**Answer/Explanation**

Answer:

Explaination: (1, 2) ∈ R, (2, 1) ∈ R, but (1, 1) ∉ R.

16. 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. [NGERT]

**Answer/Explanation**

Answer:

Explaination:

Given R = {(1, 1), (2, 2), (3, 3), (1, 2), (2, 3)} defined on R: {1, 2, 3} → {1, 2, 3}

For reflexive: As (1, 1), (2,2), (3, 3) ∈ R. Hence, reflexive

For symmetric: (1, 2) ∈ R but (2, 1) ∉ R. Hence, not symmetric.

For transitive: (1, 2) ∈ R and (2, 3) ∈R but (1, 3) ∉ R. Hence, not transitive.

17. Let A = {3, 4, 5} and relation R on set A is defined as R = {(a, b) e A x A : a – b – 10). Is relation an empty relation?

**Answer/Explanation**

Answer:

Explaination: We notice for no value of a, b s A, a-b = 10. Hence, (a, b) £ R for a, b e A. Hence, empty relation.

18. Given set A = {a, b} and relation R on A is defined as R = {(a, a), (b, b)}. Is relation an identity relation?

**Answer/Explanation**

Answer:

Explaination: Yes, as (a, a) ∈ R, for all a ∈ A..

19. Let set A represents the set of all the girls of a particular class. Relation R on A is defined as R = {(a, b) ∈ A × A : difference between weights of a and b is less than 30 kg}. Show that relation R is a universal relation.

**Answer/Explanation**

Answer:

Explaination: Let a, b ∈ A then a – b < 30 kg, always true for students of a particular class, i.e. aRb ∀ a, b ∈ A. Hence, universal relation.

20. If A = {1, 2, 3} and relation R = {(2, 3)} in A. Check whether relation R is reflexive, symmetric and transitive.

**Answer/Explanation**

Answer:

Explaination:

Not reflexive, as (1, 1) ∉ R.

Not symmetric, as (2, 3) ∈ R but (3,2) ∉ R.

Transitive, as relation R in a non empty set containing one element is transitive.

21. State the reason for the relation R in the set {1, 2, 3} given by R = {(1, 2), (2, 1)} not to be transitive. [Delhi]

**Answer/Explanation**

Answer:

Explaination: As (1, 2) ∈ R, (2, 1) ∈ R, but (1, 1) ∉ R.

22. Consider the set A containing n elements, then the total number of injective functions from set A onto itself is _____ .

**Answer/Explanation**

Answer:

Explaination: Total number of injective functions from set containing n elements to a set containing n elements is ^{n} P_{n} = n!

23. The domain of the function *f* : R → R defined by f(x) = \(\sqrt{4-x^{2}}\) is ______ .

**Answer/Explanation**

Answer:

Explaination:

[-2, 2]. For domain 4 – x² ≥ 0

⇒ 4 ≥ x²

⇒ x² ≤ 4

⇒ x² ≤ (2)²

⇒ -2 ≤ x ≤ 2, i.e. [-2, 2].

24. Let A = {a, b }. Then number of one-one functions from A to A possible are

(a) 2

(b) 4

(c) 1

(d) 3

**Answer/Explanation**

Answer:

Explaination: (a), as if n(A) = m, then possible one-one functions from A to A are m!

25. Let A = {1, 2, 3, 4} and B = {a, b, c}. Then number of one-one functions from A to B are ______.

**Answer/Explanation**

Answer:

Explaination: 0, as n(A) > n(B)

26. If n(A) = p, then number of bijective functions from set A to A are ______ ..

**Answer/Explanation**

Answer:

Explaination: p!, as for bijective functions from A to B, n(A) = n(B) and function is one-one onto.

27. The function *f *: R → R defined as *f*(x) = [x], where [x] is greatest integer ≤ x, is onto function. State true or false.

**Answer/Explanation**

Answer:

Explaination: False, as range of *f* is set of integers, i.e.

Z and range of f ⊆ co-domain R. Hence,not onto e.g. for \(\frac{1}{2}\) ∈ R (co-domain) there is no x ∈ R (domain) such that y = *f*(x) or \(\frac{1}{2}\) e∈ R has no pre-image.

28. If \(*f*(x)=\frac{x-1}{|x-1|}, x(\neq 1) \in R\) then range of ‘f’ is _______ .

**Answer/Explanation**

Answer:

Explaination:

29. If *f* : R → R be defined by *f*(x) = (3 – x^{3})^{1/3}, then find *fof*(x). [NCERT]

**Answer/Explanation**

Answer:

Explaination:

30. If f is an invertible function defined as *f*(x) = \(\frac{3x-4}{5}\), write *f*^{-1}(x).

**Answer/Explanation**

Answer:

Explaination:

31. Let A = {1, 2, 3}, B = {4, 5, 6, 7} and let *f* = {(1, 4), (2, 5), (3, 6)} be a function from A to B. State whether *f* is one-one or not. [AI 2011] *f*

**Answer/Explanation**

Answer:

Explaination: One-one, as for x_{1} ≠ x_{2}

⇒ *f*(x_{1}) ≠ *f*(x_{2}).

32. Let *f* : R → R is defined by *f* (x) = | x |. Is function f onto? Give a reason. [HOTS]

**Answer/Explanation**

Answer:

Explaination: *f* is not onto, as for some y ∈ R from co-domain, there is no x ∈ R from domain such that y = f(x), e.g. for -2 ∈ R (co-domain) there is no x ∈ R (domain) such that *f*(x) = -2, i.e. |x| = -2. Hence, not onto.

33. If f : R → R and g : R → R are given by f (x) = sin x and g(x) = 5x², find gof(x).

**Answer/Explanation**

Answer:

Explaination: gof(x) = g(f(x)) = g(sin x) = 5 sin² x.

34. Let *f* : {1, 3,4} → {1,2, 5} and g: {1,2, 5} → {1, 3} be given by *f* = {(1, 2), (3, 5), (4, 1)} and g = {(1, 3), (2, 3), (5, 1)}. Write down gof. [NCERT]

**Answer/Explanation**

Answer:

Explaination:

go*f*: {1, 3, 4} → {1, 3}.

go*f*(1) = g(*f*(1)) = g(2) = 3.

go*f*(3) = g(K3)) = g(5) = 1

go*f*(4) = g(*f*(4)) = g(1) = 3

go*f* = {(1, 3), (3, 1), (4, 3)}.

35. Prove that *f* : R → R given by *f*(x) = x^{3} + 1 is one-one function.

**Answer/Explanation**

Answer:

Explaination:

Given *f*(x) = x^{3} + 1

For x_{1} ≠ x_{2}

⇒ x_{1}^{3} ≠ x_{2}^{3}

⇒ x_{1}^{3} + 1 ≠ x_{2}^{3} + 1

⇒ *f*(x_{1}) ≠ *f*(x_{2}). Hence, one-one

36. Show that the Signum Function *f* : R → R,

one-one nor onto.

**Answer/Explanation**

Answer:

Explaination:

Range of function is {-1, 0, 1} and co-domain is set of real numbers R.

⇒ Range ⊆ co-domain.

There is at least one element in R(codomain) which is not image of any element of the domain, e.g. for 2 e R(co-domain), there is no x in domain such that *f*(x) = 2, x ∈ R.

Hence, function is not onto.

Also, let x_{1} = 2 and x_{2} = 3 then *f*(x_{1}) = 1 and *f*(x_{2}) = 1

i.e., x_{1} ≠ x_{2} ⇒ *f*(x_{1}) = *f*(x_{2}).

So, function is not one-one.

37. Given *f*(x) = sin x check if function f is one-one for (i) (0, π) (ii) (-\(\frac{π}{2}\), \(\frac{π}{2}\)).

**Answer/Explanation**

Answer:

Explaination:

38. If *f* : R → R is defined by *f*(x) = 3x + 2, define *f* (*f*(x)). [Foreign]

**Answer/Explanation**

Answer:

Explaination: *f*(*f*(x)) = *f*(3x + 2) = 3(3x + 2) + 2

= 9x+ 8.

39. Write fog, if *f* : R → R and g : R → R are given by *f*(x) = |x| and *g*(x) = |5x – 2|. [Foreign]

**Answer/Explanation**

Answer:

Explaination: (*fog*)(x) =*f*(*g*(x))

=*f*(|5x-2|) = ||5x-2||.

40. Write fog, if *f* : R → R and g : R → R are given by *f*(x) = 8x^{3} and g(x) = x^{1/3}. [Foreign]

**Answer/Explanation**

Answer:

Explaination: (*fog*)(x) = *f*(*g*(x)) = *f*(x^{1/3}) = 8(x^{1/3})^{3} = 8x.