You cannot have two vectors equal without exactly the same x component, exactly the same y component.

你不能说两个矢量相等,如果它们的 x 分量不完全相同,或者 y 分量不完全相同

耶鲁公开课 - 基础物理课程节选

Once you do that, I think it's fairly clear that it has got a y component that's given by cosine the x component is negative.

用了这个结论,我想很明显,y 分量由 cos φ 给出,它的 x 分量是负的

耶鲁公开课 - 基础物理课程节选

If you want to add two vectors, you can add the arrows by the rule I gave you or just add the components of x of the two guys to get the component of the sum and likewise for the y.

如果要把两个矢量相加,你可以用我教给你们的箭头相接的方法,或者就将二者的 x 分量相加,得到和矢量的 x 分量,y 方向也是这样

耶鲁公开课 - 基础物理课程节选