K instead of using the vertical vector k, you use the normal vector I.

你用了法向量i，而不是竖直向量。

youdao

But, when it comes to looking for the normal vector, be rest assured that the normal vector to a horizontal plane is just vertical.

但是在寻找法向量时，可以确定法向量与水平面垂直。

youdao

It's the unit vector in the vertical direction.

即竖直方向的单位向量。

youdao