Let T be the set of all triangles in a plane.
We are given that T1 R T2
⇒T1:T2 for all T1,T2∈T
Reflexive : Let T1∈T such that T1 R T1.
Then T1 R T⇒T1:T1 every triangle is similar to itself.
So, : is reflexive on T.
Symmetric: Let T1,T2∈T such that T1 R T2
Then T1 R T2⇒T1:T2⇒T2:T1
So, r is symmetric on T.
Transitive: Let T1,T2,T3∈T such that T1 R T2,T2 R T3.
Then, T1 R T2⇒T1:T2 and T2:T3 implies that T1:T3
So, R is transitive on T.
Hence, R is an equivalence relation on T.