I4.4 In figure 3 , the area of the square $A B C D$ is equal to $d$. If $E, F, G, H$ are the mid-points of $A B, B C, C D$ and $D A$ respectively and $E F=c$, find the value of $d$.
Area of $E F G H=c^{2}=2^{2}=4$
Area of $A B C D=2 \times$ area of $E F G H=8$ $\Rightarrow d=8$