Question 7
Circles $A,B$ of radius one centimetre touch externally. Equal-radius circles $P,Q,R,S$ have the shown external tangencies: $P$ touches $A,B,Q,S$; $Q$ touches $P,B,R$; $R$ touches $A,B,Q,S$; $S$ touches $P,A,R$. Find their common radius in centimetres.
