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
Only when several component vectors are on a straight line will the length of the vector representing their sum be equal to the sum of the lengths of these components or their differences
以上为机器翻译结果，长、整句建议使用 人工翻译 。