当c≠0时,有 (a+b)c=c Prje(a+b =cl(Prja+Prj b) 引cPja+cPjb a·c+b.c 证毕 D合当c≠0时，有 证毕 (a b) c c Prj (a b) +  =| | c + c (Prj a Prj b) = c + c | | =| c | Prj c a + cPrj c b = ac + bc