K is the difference of vectors, and the result is still vectors; |△r| is the modulus length of △r, a numerical value and a scalar. In college physics, the vector diameter is expressed by r=xi+yj+zk, where I, J and K are unit vectors in the positive direction of X, Y and Z axes respectively. △r represents the difference between the diameters of two vectors, that is, △ r = R2-r1= (x2-x1) i+(y2-y1) j+(z2-z1) k, which is a vector. And |△r| represents the module length of vector △r, that is |△ r | = √ [(x2-x1) _+(y2-y1) _+(z2-z1) _], which is a numerical value.