Affine, Euclidean, and projective geometry

4901. Prove that a b ¯ + b c ¯ = a c ¯ https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203756454/84bc859e-388a-4821-81f7-7fb447dc7024/content/eq1432.tif"/> for any points a, b, c of an affine space.