| Let us define a relation R in R as

R = {(a,b) : a³ ≥ b³}

It is clear that (a,a) ϵ R as a³ = a³

⇒ R is reflexive.

Now, (2,1) ϵ R

But (1,2) ∉ R

⇒ R is not symmetric.

Now, let (a,b) (b,c) ϵ R

⇒ a³≥ b³ and b³≥ c³

⇒ a³≥ c³

⇒ (a,c) ϵ R

⇒ R is transitive.

Therefore, relation R is reflexive and transitive but not symmetric.