$解：如圖，過(guò)點(diǎn)?B?作?BE⊥CD?于點(diǎn)?E，?連接?BD?$
$∵?AB=4m，??C?為?AB?的中點(diǎn)$
$∴?BC=2m?$
$∵?∠BCD=60°?$
$∴?BE=sin 60°×BC=\sqrt 3，??CE=cos 60°×BC=1?$
$設(shè)?DE=x?$
$?CD=x+1，??BD=x+1-0.5=x+0.5?$
$∵?BE^2+DE^2=BD^2?$
$∴?x^2+(\sqrt 3)^2=(x+0.5)^2?$
$?x=2.75?$
$?CD=x+1=3.75m?$
