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∵∠B,∠C的平分線相交於點O,∴∠OBD=∠OBC,∠OCE=∠OCB,∵DE‖BC,∴∠DOB=∠OBC,∠EOC=∠OCB,∴∠OBD=∠DOB,∠EOC=∠ECO,∴OD=BD,OE=CE,∴DE=OD+OE=BD+EC,∵BD+EC=5,∴DE=5.
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