Example 7.1.1 a11 0 0 b11 0 0 0 a22 0 0 b22 0 0 0 ann 0 0 bnn a11b11 0 0 0 a22b22 。。 0 0 0 annbnn Proof: cg=∑a={0 bi ifi=j k=1 fi卡j (Tongji University) 6/14. Example 7.1.1 .. a11 0 . . . 0 0 a22 . . . 0 ... ... ... 0 0 . . . ann   b11 0 . . . 0 0 b22 . . . 0 ... ... ... 0 0 . . . bnn  =  a11 b11 0 . . . 0 0 a22 b22 . . . 0 ... ... ... 0 0 . . . annbnn  . Proof: cij = ∑n k=1 aik bkj = { aii bii if i = j 0 if i ̸= j (Tongji University) Linear Algebra 6 / 14