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 angled triangle $T$ with sides $3, 4$ and $5$?