At 23:34, why should a_1 and a_2 both be perpendicular to  the vector b? How can that be even possible?####[['NULL', 'Mathematical_Related_Concept', 'NEG'], ['a_1', 'Mathematical_Related_Concept', 'NEG'], ['a_2', 'Mathematical_Related_Concept', 'NEG'], ['vector b', 'Mathematical_Related_Concept', 'NEG']]