\frac{E C}{C A} = \frac{S_{1} S_{2}}{A S_{1}} = \frac{S_{1} S_{2}}{C S_{1} - C A} ⟹ E C = \frac{S_{1} S_{2}}{\frac{C S_{1}}{C A} - 1} = \frac{S_{1} S_{2}}{2} = 10 c m

${\dfrac{EC}{CA}=\dfrac{S_1S_2}{AS_1}=\dfrac{S_1S_2}{CS_1-CA}\implies EC= \dfrac{S_1S_2}{\frac{CS_1}{CA}-1}=\dfrac{S_1S_2}{2}=10\mathrm{cm}}$