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因為ABCD是菱形,連接PD,則PB=PD PB+PE=PD+PE,連接DE,當點P不在DE上時,構成三角形PDE,根據三角形兩邊之和大於第三邊的原理,PD+PE>DE所以,只有當點P在DE上時,PD+PE=DE為最小值.因為∠ADC=120°,所以∠BCD=60°,連…
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