-->

Relations And Functions

Question
CBSEENMA12035788

If the sum of lengths of the hypotenuse and a side of a right angled triangle is given, show that the area of the triangle is maximum, when the angle between them is π/3.

Solution

Let AC = y and BC = x and x + y = k (k is a constant)
Suppose θ be the angle between BC and AC.
Let A be the area of a triangle. Then,

Again space differentiating space left parenthesis 2 right parenthesis comma space we space get
2 open parentheses dA over dx close parentheses squared space plus 2 space straight A fraction numerator straight d squared straight A over denominator dx end fraction space equals space fraction numerator 2 straight k squared minus 12 kx over denominator 4 end fraction space... space left parenthesis 3 right parenthesis
Put space dA over dx space equals space 0 space and space straight x space equals space straight k over 3 space in space left parenthesis 3 right parenthesis comma space we space have
fraction numerator straight d squared straight A over denominator dx squared end fraction space equals space minus fraction numerator straight k squared over denominator 4 straight A end fraction space less than 0
Thus comma space straight A space is space maximum space when space straight x space equals space straight k over straight x
when space straight x space equals space straight k over 3 comma space straight y space equals space straight k minus straight k over 3 space equals space fraction numerator 2 straight k over denominator 3 end fraction
therefore space cos space straight theta space equals space straight x over straight y
rightwards double arrow space cos space straight theta space equals space fraction numerator straight k divided by 3 over denominator 2 straight k divided by 3 end fraction space equals space 1 divided by 2
rightwards double arrow space cos space straight theta space equals space cos space straight pi divided by 3
straight theta space equals space straight pi over 3

Some More Questions From Relations and Functions Chapter

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’. 

If R and R’ are symmetric relations on a set A, then R ∩ R’ is also a sysmetric relation on A.

Show that the union of two symmetric relations on a set is again a symmetric relation on that set.