20.1    Inverse[{{1,4},{-4,1}}]

20.6    b) solve Determinant[{{k,1,2},{0,2,0},{3,4,k}}]=0

20.7    a) Inverse[{{0,1,2},{3,0,1},{-2,3,0}}]
        b) Inverse[{{1,1,1,1},{0,1,1,1},{0,0,1,1},{0,0,0,1}}]

20.8    b) Inverse[{{2,-1},{3,2}}]
        e) Inverse[{{2,4,0},{3,4,-2},{-1,1,2}}]

20.11   a) LinearSolve[{{4,2,6},{1,-2,3},{1,-1,3}},{-3,-1,-3/2}]

20.12	b) {{-2,1},{1,0}}.Inverse[{{1,2},{3,7}}]
        e) Inverse[{{0,4,1},{-2,1,3},{2,4,9}}].{{4,6},{-3,7},{8,22}}
        g) {{-2,24,3}}.Inverse[{{-1,2,1},{0,3,0},{1,2,-2}}]

20.13   b) Inverse[{{2,-3,1},{4,-5,2},{5,-7,3}}].{{2,0,-2},{18,12,9},{23,15,11}}.Inverse[{{9,7,6},{1,1,2},{1,1,1}}]
        d) Inverse[{{1,2},{4,-1}}].{{2,0},{1,-1}}.Inverse[{{0,5},{1,1}}]