[a + b-c] . [(a-b)x (b-c) = (a + b - c). [axb-axc-bxb + bxc] = a. (a x b) - a. (a x c) + a. (b x c) + b. (a x b) - b (a x c) + b. (b x c) - c. (a x b) + c. (a x c) - c.(b x c) = a. (b x c) - b. (axe) - c. (a x b) = [a b c] - [b a c] - [c a b] =[a b c] + [a b c] - [a b c] = [a b c] = a. (b x c)