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Let $\mathrm{A}$ be the set of all students of a boys school. Show that the relation $\mathrm{R}$ in A given by $\mathrm{R} =\{(a, b): \mathrm{a} $ is sister of $\mathrm{b}\}$ is the empty relation and $\mathrm{R} ^{\prime}=\{(a, b)$ $:$ the difference between heights of $\mathrm{a}$ and $\mathrm{b}$ is less than $3\,\mathrm{meters}$ $\}$ is the universal relation.
Solution
since the school is boys school, no student of the school can be sister of any student of the school. Hence, $\mathrm{R} =\phi,$ showing that $\mathrm{R}$ is the empty relation. It is also obvious that the difference between heights of any two students of the school has to be less than $3\,\mathrm{meters}$. This shows that $\mathrm{R}^{\prime}=\mathrm{A} \times \mathrm{A}$ is the universal relation.
