I think this becomes more clear when we simplify the example by making the point (x, y) on the line in which the line A to the line BC is perpendicular to the line BC. In this case, we'll see that alpha = dist(point, BC) / dist(A, BC). This makes it more clear that alpha is actually represents a proportion of the total perpendicular distance between A and BC
artpomm
Slides definitely should be changed to reflect the above ^ it's not an absolute distance.
I think this becomes more clear when we simplify the example by making the point (x, y) on the line in which the line A to the line BC is perpendicular to the line BC. In this case, we'll see that alpha = dist(point, BC) / dist(A, BC). This makes it more clear that alpha is actually represents a proportion of the total perpendicular distance between A and BC
Slides definitely should be changed to reflect the above ^ it's not an absolute distance.