(vector AB/ modulus of vector AB): is the unit vector of vector AB.

(modulus of vector AB/modulus of vector AB+ modulus of vector AC): a vector obtained by adding the unit vector of vector AB and the unit vector of vector AC.

(modulus of vector AB /loud AB+ modulus of vector AC) × vector BC =0: the vector obtained by adding the unit vector of vector AB and the unit vector of vector AC has the same direction as that of vector BC, so the area of the parallelogram formed by these two vectors is zero.

The diagonal of the parallelogram formed by the unit vector of vector AB and the unit vector of vector AC is the same as the direction of vector BC.