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Q.Check whether the relation RR in R\mathbb{R} defined by R={(a,b):a≤b3}R = \{(a, b) : a \le b^3\} is reflexive, symmetric and transitive.

Karnataka PUCKarnataka II PUC Board 2025Subjective· 3mImportance★★★★★
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Test each property with a counterexample: R={(a,b):a≤b3}R=\{(a,b):a\le b^3\} on R\mathbb{R} is neither reflexive, nor symmetric, nor transitive.

Reflexive? RR is reflexive if (a,a)∈R(a,a)\in R, i.e. a≤a3a \le a^3, for every real aa.

Take a=12a = \dfrac{1}{2}: then a3=18a^3 = \dfrac{1}{8}, and

12≤18\frac{1}{2} \le \frac{1}{8}

is false. So the condition fails for at least one aa. RR is not reflexive.

Symmetric? RR is symmetric if (a,b)∈R⇒(b,a)∈R(a,b)\in R \Rightarrow (b,a)\in R, i.e. a≤b3⇒b≤a3a\le b^3 \Rightarrow b \le a^3.

Take a=1, b=2a = 1,\ b = 2: then a≤b3a \le b^3 means 1≤81 \le 8, which is true, so (1,2)∈R(1,2)\in R. But b≤a3b \le a^3 means 2≤12 \le 1, which is false, so (2,1)∉R(2,1)\notin R. RR is not symmetric. …

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