下圖的梯形 ABCD 的對角線 AC 和 BD 相互垂直, 已知 AD = 3, AC = 9, BD = 12, 則 BC 的長度為____.---**Question Number:** 3 **Question Stem:** 下圖的梯形 ABCD 的對角線 AC 和 BD 相互垂直, 已知 AD = 3, AC = 9, BD = 12, 則 BC 的長度為____. **Given Information:** * Figure is a trapezoid ABCD. * Diagonals AC and BD are perpendicular to each other (AC ⊥ BD). * AD = 3 * AC = 9 * BD = 12 **To Find:** The length of BC. **Figure Description:** * Type: Geometric figure (Trapezoid with diagonals). * Elements: * A quadrilateral labeled ABCD, representing a trapezoid. * Vertices are labeled A (top left), D (top right), B (bottom left), C (bottom right). AD is the top base and BC is the bottom base (implied by the standard notation for a trapezoid and visual appearance). * Diagonals AC and BD are drawn inside the trapezoid, connecting opposite vertices. * The diagonals intersect at a point (not labeled). * A right angle symbol is shown at the intersection point of diagonals AC and BD, indicating that they are perpendicular. * Lines: Segments AB, BC, CD, DA (sides of the trapezoid), AC (diagonal), BD (diagonal).