# PRACTICAL PROBLEMS ON RELATIONS AND FUNCTIONS IN SET THEORY

Problem 1 :

An open box is to be made from a square piece of material, 24 cm on a side, by cutting equal squares from the corners and turning up the sides as shown. Express the volume V of the box as a function of x.

Solution :

Since the original shape is square, length of all sides will be equal.

length  =  width  =  24 - 2x

height  =  x

Volume of cuboid  =  length ⋅ width ⋅ height

=  (24 - 2x) ⋅ (24 - 2x) ⋅ x

=  x(24 - 2x)2

=  x [242 - 2(24) (2x) + (2x)2]

=  x [576 - 96x + 4x2]

V (x)  =  4x- 96x+ 576x

Hence the volume of the cuboid is 4x- 96x+ 576x.

Problem 2 :

A function f is defined by f (x) = 3−2x . Find x such that f (x2) = (f (x))2 .

Solution :

Given that :

f (x)  =  3 − 2x

f (x2)  =  3 − 2x2 ------(1)

(f (x))2  =  (3 − 2x)2

=  32 - 2(3)(2x) + (2x)2

(f (x))2  =  9 - 12x + 4x2  ----(2)

(1)  =  (2)

3 − 2x2  =  9 - 12x + 4x2

4x2 + 2x2 - 12x + 9 - 3  =  0

6x2 - 12x + 6  =  0

x2 - 2x + 1  =  0

(x - 1)2  =  0

x - 1  =  0 (or)  x - 1  =  0

x  =  1 (or) x  =  1

Problem 3 :

A plane is flying at a speed of 500 km per hour. Express the distance d travelled by the plane as function of time t in hours.

Solution :

Time  =  Distance / speed

Speed of plane  =  500 km per hour

Distance travelled  =  d

time taken  =  t in hours

t  =  d/500

d  =  500t

Hence the required distance is 500 t.

Problem 4 :

The data in the adjacent table depicts the length of a woman’s forehand and her corresponding height. Based on this data, a student finds a relationship between the height (y) and the forehand length(x) as y = ax +b , where a, b are constants.

(i) Check if this relation is a function.

(ii) Find a and b.

(iii) Find the height of a woman whose forehand length is 40 cm.

(iv) Find the length of forehand of a woman if her height is 53.3 inches.

Solution :

y = ax + b

(i)  For every values of x, we get different values of y.Hence it is a function.

(ii)

 y = a x + bx = 45.5, y = 65.565.5  =  a(45.5) + b65.5  =  45.5 a + b --(1) y = a x + bx = 35, y = 5656  =  a(35) + b56  =  35 a + b --(2)

By solving these two equation, we get a and b

(1) - (2)

(45.5 a + b) - (35a + b)  =  65.5 - 56

45.5 a - 35a + b - b  =  9.5

10.5a  =  9.5

a  =  9.5/10.5

a  =  0.90

Substitute a = 0.90 in (1),

45.5(0.90) + b  =  65.5

b  =  65.5 - 40.95

b  =  24.5

y = 0.9x + 24.5

(iii) Find the height of a woman whose forehand length is 40 cm.

y = ? if x = 40

y = 0.9(40) + 24.5

y = 60.5

Height of woman is 60.5 inches.

(iv)  x = ? if y = 53.3

53.3 = 0.9x + 24.5

53.3 - 24.5  =  0.9x

28.8/0.9  =  x

x  =  32

Length of forehand is 32 inches.

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