28.1  a) Cross[(1,0,0),(1,-5,-2)]
         -(0,2,-5).(3,-1,1)
      b) Cross[(0,1,0),(4,1,-3)]
         -(-3,0,-4).(3,4,0)
      c) Cross[(0,0,1),(2,5,7)]
         -(-5,2,0).(1,3,4)

28.2  a) Cross[(1,4,0),(3,-2,1)]
         -(4, -1, -14).(0,0,0)
      b) Cross[(2,1,1),(-4,-3,-1)]
         -(2, -2, -2).(-1,7,-6)
      c) Cross[(1,0,-3),(-1,1,0)]
         -(3, 3, 1).(3,-1,2)

28.3  a) -(2, 3, -5).(0,0,6)
      b) -(-1, 4, 7).(0,3,0)

28.4     LinearSolve[{{5,0,-1},{2,-1,-4},{0,3,2},{3,4,5}},{-3,-5,1,3}]

28.5     -(1, -2, 3).(1,-2,1)

28.7  a) arccos(|(-2,4,-1).(7,3,-2)|/sqrt(((-2,4,-1).(-2,4,-1))*((7,3,-2).(7,3,-2))))
      b) arccos(|(0,1,0).(0,3,-3)|/sqrt(((0,1,0).(0,1,0))*((0,3,-3).(0,3,-3))))

28.8     Cross[(3,-2,1),(1,0,-2)]
         Cross[(4,7,2),(1,-2,1)]
         -(11,-2,-15).(2,2,1)

28.10 a) |(1,2,1).(1,2,2)-10|/sqrt((1,2,2).(1,2,2))
      b) |(0,0,0).(15,-10,6)-190|/sqrt((15,-10,6).(15,-10,6))
      c) |(2,8,5).(1,-2,-2)-1|/sqrt((1,-2,-2).(1,-2,-2))

28.11    :(

28.12 a) :(
      b) :(
      d) :(

28.13    Cross[(3,7,-2),(2,-6,7)]

28.14    Cross[(2,-3,4),(0,1,0)]

28.15 a) arcsin(|(4,3,1).(3,5,-1)|/sqrt(((4,3,1).(4,3,1))*((3,5,-1).(3,5,-1))))
         Cross[(4,3,1),(3,5,-1)]
      b) arcsin(|(2,4,3).(3,-3,2)|/sqrt(((2,4,3).(2,4,3))*((3,-3,2).(3,-3,2))))
      c) Cross[(3,5,-7),(2,-1,1)]
         arcsin(|(-2, -17, -13).(5,0,-1)|/sqrt(((-2, -17, -13).(-2, -17, -13))*((5,0,-1).(5,0,-1))))
         LinearSolve[{{3,5,-7},{2,-1,1},{5,0,-1}},{-16,6,4}]