MathNet 1982 — Part 01 Question 9

In the diagram, $OB_{i}$ is parallel and equal in length to $A_{i}A_{i+1}$ for $i=1,2,3$ and $4$ ($A_{5}=A_{1}$). Show that the area of $B_{1}B_{2}B_{3}B_{4}$ is twice that of $A_{1}A_{2}A_{3}A_{4}$.