只有当几个分向量在一直线上那么表示其和的向量长度才会等于这些分量的长度和或它们的差
It's only when the component vectors are on the line that the length of the vector that represents their sum is equal to the length of the components or their difference
以上为机器翻译结果，长、整句建议使用 人工翻译 。
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