Because ABC is on the same plane, AB and AC are perpendicular to N vector at the same time, AB vector and AC vector will intersect and have cross product.

The result of the outer product (cross product) of two vectors A 1 and A2 is vector b,

The direction of vector B is perpendicular to the original two vectors a 1, a2, and the module length is | b | =| a1| | A2 | * sin α.