Giải thích các bước giải:

Ta có: $CD//AB(\perp AD)$

$\to \dfrac{AD}{DE}=\dfrac{BC}{CE}$

$\to \dfrac{AD^2}{DE^2}=\dfrac{BC^2}{CE^2}$

$\to \dfrac{AD^2}{BC^2}=\dfrac{DE^2}{CE^2}$

$\to \dfrac{AD^2}{BC^2}=\dfrac{CE^2-CD^2}{CE^2}$

$\to \dfrac{AD^2}{BC^2}=1-\dfrac{CD^2}{CE^2}$

$\to \dfrac{AD^2}{BC^2}=1-\dfrac{AD^2}{CE^2}$

$\to \dfrac{AD^2}{BC^2}+\dfrac{AD^2}{CE^2}=1$

$\to \dfrac1{BC^2}+\dfrac1{CE^2}=\dfrac1{AD^2}$