Properties of+and· Proof(continued). (4)We want to show u+（-1)u=0 () From S4,(1.)uu,so LHS(*)=(1)u+(-1)u=(1+(-1)u 0-u=0rom(3). 4口+++左+4生+定QCProperties of + and · Proof (continued). (4) We want to show u + (−1)·u = 0 (∗) From S4, (1·)u = u, so LHS(∗) = (1)·u + (−1)u = (1 + (−1))u 0·u = 0 from (3)