例4 ax, tax+.+.x=b 12 nn a21x,+a222+.+arnn=b, ax1+a,,+…+a.=b n nn n 12 n 令A 2n b 2 mn 则方程组可写作A=b例4        + + + = + + + = + + + = m m m n n m n n n n a x a x a x b a x a x a x b a x a x a x b     1 1 2 2 2 1 1 2 2 2 2 2 1 1 1 1 2 2 1 1               =               =               = m m m n n m n n b b b b x x x x a a a a a a a a a A          2 1 2 1 1 2 2 1 2 2 2 1 1 1 2 1 令 , , 则方程组可写作Ax = b