For three points to be collinear, the area of the triangle formed by the three points should be zero. ∴ Area of D formed by the given three points= \(rac{1}{2}\) [a((c + a) – (a + b)) + b((a + b) – (b + c)) + c((b + c) – (c + a))]= \(rac{1}{2}\) [a(c – b) + b(a – c) + c(b – a)] = \(rac{1}{2}\) [ac – ab + ba – bc + cb – ca] = 0.Hence (a, b + c), (b, c + a) and (c, a + b) are collinear.