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因為角ACD=60度,CE是三角形ACD的角平分線, 所以角ECD=角ACD的一半=30度, 因為角BCD=40度,CF是三角形BCD的角平分線, 所以角FCD=角BCD的一半=20度, 所以角ECF=角ECD+角FCD=50度.
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