Problem
\( \mathrm{ABC} \) is a right angled triangle at \( \mathrm{B}, \overline{B D} \perp \overline{A C} \) then \( (B D)^{2}=A D \times \ldots \)
Solution
Step 1 ：\( (AC)^{2}=(AB)^{2}+(BC)^{2} \)
Step 2 ：\( (AD)^{2}+(BD)^{2}=(DC)^{2}+(BD)^{2} \)
Step 3 ：\( (BD)^{2}=AD \times DC \)
