Example 1 Let A be the set of all students of a boys school Show that the relation R
in A given by R = {(a, b) : a is sister of b} is the empty relation and R′ = {(a, b) : the
difference between heights of a and b is less than 3 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, R = φ, showing that R is the empty relation