Solution: As shown in the figure, the side of the cone is fanned out, and the corresponding arc length is the circumference of the bottom circle 2 π r. If you move to D along the side of the cone, the shortest distance is AD.

Let ∠ASD=α, then 2πR=|α|×3R, ∴|α|=2π3.

∴AD=9R2+9R24？ 2×3R×3R2×(？ 12)=372R