如图，求MD的长---**Extraction Content:** **Question Stem:** 例 6: 如图，两圆相交于 C, D, AB 为公切线，若 AB=12, CD=9, 求 MD 的长。 **Provided Information:** AB = 12 CD = 9 Seek the length of MD. **Chart/Diagram Description:** Type: Geometric diagram. Main Elements: - Two circles are shown, overlapping and intersecting at two points, labeled C (upper intersection) and D (lower intersection). - A straight line segment, labeled AB, is tangent to both circles. Point A is on the left circle, and point B is on the right circle. AB is positioned below the circles. - A straight line segment, labeled CD, connects the two intersection points of the circles. CD appears to be a vertical line segment, perpendicular to the tangent line AB. - A point M is labeled on the tangent line AB, between points A and B. M appears to be the foot of the perpendicular from D to AB. - A straight line segment, labeled MD, connects point D to point M. This segment MD appears vertical, perpendicular to AB. - Labels: Points A, B, C, D, and M are clearly labeled. The line segments AB, CD, and MD are implied by the points. **Relative Position and Direction:** - The left circle is larger than the right circle. - C is above D. - CD is approximately a vertical line segment. - AB is a horizontal line segment below the intersection points. - M is on AB, directly below D. - MD is approximately a vertical line segment, perpendicular to AB.