

InterviewSolution
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Test whether the following relations R1, R2 and R3 are (i) reflexive (ii) symmetric and (iii) transitive :R3 on R defined by (a, b) ϵ R3⇔ a2 – 4 ab + 3b2 = 0. |
Answer» Here, R1, R2, R3, and R4 are the binary relations. So, recall that for any binary relation R on set A. We have, R is reflexive if for all x ∈ A, xRx. R is symmetric if for all x, y ∈ A, if xRy, then yRx. R is transitive if for all x, y, z ∈ A, if xRy and yRz, then xRz. So, using these results let us start determining given relations. We have R3 on R defined by (a, b) ∈ R3⇔ a2 – 4ab + 3b2 = 0 Check for Reflexivity: ∀ a ∈ R, (a, a) ∈ R3 needs to be proved for reflexivity. If (a, b) ∈ R3, then we have a2 – 4ab + 3b2 = 0 Replace b by a, we get a2 – 4aa + 3a2 = 0 ⇒ a2 – 4a2 + 3a2 = 0 ⇒ –3a2 + 3a2 = 0 ⇒ 0 = 0, which is true. ⇒ (a, a) ∈ R3 So, ∀ a ∈ R, (a, a) ∈ R3 ∴ R3 is reflexive. Check for Symmetry: ∀ a, b ∈ R If (a, b) ∈ R3, then we have a2 – 4ab + 3b2 = 0 ⇒ a2 – 3ab – ab + 3b2 = 0 ⇒ a (a – 3b) – b (a – 3b) = 0 ⇒ (a – b) (a – 3b) = 0 ⇒ (a – b) = 0 or (a – 3b) = 0 ⇒ a = b or a = 3b …(1) Replace a by b and b by a in equation (1), we get ⇒ b = a or b = 3a …(2) Results (1) and (2) does not match. ⇒ (b, a) ∉ R3 ∴ R3 is not symmetric. Check for Transitivity: ∀ a, b, c ∈ R If (a, b) ∈ R3 and (b, c) ∈ R3 ⇒ a2 – 4ab + 3b2 = 0 and b2 – 4bc + 3c2 = 0 ⇒ a2 – 3ab – ab + 3b2 = 0 and b2 – 3bc – bc + 3c2 ⇒ a (a – 3b) – b (a – 3b) = 0 and b (b – 3c) – c (b – 3c) = 0 ⇒ (a – b) (a – 3b) = 0 and (b – c) (b – 3c) = 0 ⇒ (a – b) = 0 or (a – 3b) = 0 And (b – c) = 0 or (b – 3c) = 0 ⇒ a = b or a = 3b And b = c or b = 3c What we need to prove here is that, a = c or a = 3c Take a = b and b = c Clearly implies that a = c. [∵ if a = b, just substitute a in place of b in b = c. We get, a = c] Now, take a = 3b and b = 3c If a = 3b b = a/3 Substitute b = a/3 in b = 3c. We get a/3 = 3c ⇒ a = 9c, which is not the desired result. ⇒ (a, c) ∉ R3 ∴ R3 is not transitive. |
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