So, we want to decompose the position vector into a sum of simpler vectors.
故我们是把位置向量，分解为简单向量的和。
Only if the component vectors are on a straight line will the length of the vector for the sum be equal to the sum of the lengths of these components or to their difference.
只有当几个分矢量在一直线上，那么表示其和的矢量长度才会等于这些分量的长度和或它们的差。
The operation of finding the sum of two vectors is called vector addition .
求两个向量和的运算，叫做向量的加法。
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