Could someone give me a clue why use A(T)AX(hat)=A(T)b to solve [C,D] at 21:05 ? To my understanding, AX(hat) cannot be b if we fit a line which points are P1, P2, P3 instead of b1, b2, b3 or [1 2 2]_t?####[['A(T)AX(hat)=A(T)b', 'Mathematical_Related_Concept', 'NEU']]