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△DEC是等腰三角形 證明:因為CD是等腰三角形ABC的底邊AB上的高, 所以D為AB中點 又DE平行BC, 則DE=BC的一半,且E為AC中點,即EC=AC的一半 又因為AC=BC 所以DE=EC 所以△DEC是等腰三角形
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