Section 1.1 & 1.2: Types of Relations

Show that the relation \(R\) defined in the set \(A\) of all polygons as
\[R = \{(P_1, P_2) : P_1 \text{ and } P_2 \text{ have same number of sides}\}\]
is an equivalence relation. What is the set of all elements in \(A\) related to the right angle triangle \(T\) with sides 3, 4 and 5?