Check whether the relation R on R defined by R = {(a, b) : a ≤ b3} is reflexive, symmetric or transitive.

We have the set of real numbers, R.


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.


We have


R = {(a, b): a ≤ b3}


Check for Reflexivity:


For a R


If (a, a) R,


a ≤ a3, which is not true.


Say, if a = – 2.


a ≤ a3


– 2 ≤ – 8


–2 ≤ –8, which is not true as – 2 > – 8.


Hence, (a, a) R


So, a R, then (a, a) R.


R is not reflexive.


R is not reflexive.


Check for Symmetry:


a, b R


If (a, b) R


a ≤ b3


Replace a by b and b by a, we get


b ≤ a3


[Take a = –2 and b = 3.


a ≤ b3


–2 ≤ 33


–2 ≤ 27, which is a true statement.


Now, b ≤ a3


3 ≤ (–2)3


3 ≤ –8, which is not a true statement as 3 > –8]


(b, a) R


So, if (a, b) R, then (b, a) R


a, b R


R is not symmetric.


R is not symmetric.


Check for Transitivity:


a, b, c R


If (a, b) R and (b, c) R


a ≤ b3 and b ≤ c3


a ≤ c3 or not.


Let us check.


Take a = 3, and .


a ≤ b3




3 ≤ 3.37, which is true.


b ≤ c3




1.5 ≤ 1.728


a ≤ c3




3 ≤ 1.728, which is not true as 3 > 1.728.


Hence, (a, c) R.


So, if (a, b) R and (b, c) R, then (a, c) R.


a, b, c


R is not transitive.


R is not transitive.


Hence, R is neither reflexive, nor symmetric, nor transitive.


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