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Home  >>  CBSE XII  >>  Math  >>  Relations and Functions
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Given a Relation $R$ in a set of Real Numbers defined as $(R) =\{(a, b: (a \leq b^2 )\}$. Is the Relation reflexive, transitive or symmetric?

$\begin{array}{1 1} \text{Not Reflexive, Not Symmetric, Not Transitive} \\ \text{Reflexive, Symmetric, Transitive} \\ \text{Only Reflexive} \\ \text{Only Transitive} \end{array} $

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Toolbox:
  • A relation R in a set A is called reflexive. if $(a,a) \in R\;for\; all\; a\in A$
  • A relation R in a set A is called symmetric. if $(a_1,a_2) \in R\;\Rightarrow \; (a_2,a_1)\in R \;$ for $\;a_1,a_2 \in A$
  • A relation R in a set A is called transitive. if $(a_1,a_2) \in\; R$ and $(a_2,a_3)\in R \Rightarrow \;(a_1,a_3)\in R\; $for all $\; a_1,a_2,a_3 \in A$
  • In order to prove the result, we must always solve for the general case. In order to disprove some result, it is sufficient to prove it for one specific ordered pair that does not satisfy the relation.
Given in a set of real numbers, the relation $R=\{(a,b):a\leq b^2\}$:
Let $a = \frac{1}{3} \; \rightarrow \ a^2 = \frac{1}{3} \times \frac{1}{3} = \frac{1}{9}$
Clearly in this case, since $\frac{1}{3} \not \leq (\frac{1}{3})^2$, i.e., $\frac{1}{3} \not \leq \frac{1}{9}$,
Therefore since $(\frac{1}{3}.\frac{1}{3}) \not \in R$, $R$ is not reflexive.
Let $a = 1$ and $b = 3 \rightarrow \; a \leq b^2 \rightarrow 1 \leq 3 \times 3 \rightarrow 1 \leq 9$.
However, $a^2 = 1 \times 1 = 1 \rightarrow 3 \not \leq 1$ i.e., $ b \not \leq a^2 $.
Therefore while $(x,y) \in R$, $(y,x) \not \in R$. Hence is $R$ is not symmetric.
Let $a = 2$ and $b = -3 \; \rightarrow \ b^2 = -3 \times -3 = 9 \rightarrow 2 \leq 9 \rightarrow a \leq b^2$.
Let $b = -3$ and $c =1 \; \rightarrow \ c^2 = 1 \times 1 = 1 \rightarrow -3 \leq 1 \rightarrow b \leq c^2$.
However $a =2$ and $c =1 \; \rightarrow \ c^2 = 1 \times 1 = 1 \rightarrow 2 \not \leq 1 \rightarrow a \not \leq c^2$.
Hence $R$ is not transitive.
answered Mar 8, 2013 by balaji.thirumalai
 

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