+21978 ABCD is a square ...
\(\text{Let CDA $=$ ADF $=90^{\circ} $ } \\ \text{Let ADE $=$ AED $=\varphi $ } \\ \text{Let EDF $=$ DFE $=$ FED $=$ ADF $-$ ADE $=90^{\circ}-\varphi $} \\ \text{Let AEF $=$ AED $+$ FED $=\varphi + 90^{\circ}-\varphi =90^{\circ} $} \)
\(\mathbf{AEF = 90^{\circ}}\)
+21978 ABCD is a square ...
\(\text{Let CDA $=$ ADF $=90^{\circ} $ } \\ \text{Let ADE $=$ AED $=\varphi $ } \\ \text{Let EDF $=$ DFE $=$ FED $=$ ADF $-$ ADE $=90^{\circ}-\varphi $} \\ \text{Let AEF $=$ AED $+$ FED $=\varphi + 90^{\circ}-\varphi =90^{\circ} $} \)
\(\mathbf{AEF = 90^{\circ}}\)
