$解:過C作CE⊥AB于E,如圖: $
$∵∠BAD=45°$
$∴△ABD是等腰直角三角形$
$∴∠ABD=45°,AD=BD=200,AB=200\sqrt{2}(米)$
$∴△BCE是等腰直角三角形$
$∴∠BCE=∠EBC=45°,BE=CE$
$∵∠ACB=90°-∠DAC=75°$
$∴∠ACE=∠ACB-∠ECB=30°$
$設(shè)AE=x米,則AC=2x米$
$∴CE=\sqrt{3}AE=\sqrt{3}x米,BE=AB-AE=(200\sqrt{2}-x)米$
$∴\sqrt{3}x=200\sqrt{2}-x,解得x=100\sqrt{6}-100\sqrt{2}$
$∴CE=\sqrt{3}x=(300\sqrt{2}-100\sqrt{6})米$
$∴BC=\sqrt{2}CE=(600-200\sqrt{3})米$
$∴CD=BC-BD=400-200\sqrt{3}≈54(米)$
$∴CD的長度約為54米$