例3设 D lk II lk nn lk D=: D nn 证明D=DD2例3 设 k kk n n n k n k kk k c c b b c c b b a a a a D 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 = k kk k a a a a D 1 11 1 1 = n nn n b b b b D 1 11 1 2 = 证明 D = D1 D2