1.5   a) 8*6*cos(2*pi/3)
      b) -(2^2+3^2-4^2)/2
      c) (4^2+12^2-10^2)/2
      d) 1*1*cos(135*pi/180)
      f) 3*1*cos(0)

1.6   a) (2,-3).(4,2)
      b) (2,-3,1).(4,2,-5)
      c) (7,-4,2).(3,1,0)
      d) (0,0,0).(-2,6,0)

1.7   b) (3,1,-2).(1,-4,-5)

1.8   sqrt(4^2+2^2+6^2+2*(2*4+2*6+4*6)*cos(pi/3))

1.9   x+3y-14=0

1.10  18*x-8y-53=0

1.13  a) 6-4 = sqrt(10)*sqrt(20)*cos(alpha)
      b) -1+30+15 = sqrt(62)*sqrt(35)*cos(alpha)
      c) 2-4-4 = sqrt(9)*sqrt(21)*cos(alpha)

1.14  a) arccos((2,-7).(5,2)/sqrt(((2,-7).(2,-7))*((5,2).(5,2))))
      b) arccos((4,16,-12).(-6,-24,18)/sqrt(((4,16,-12).(4,16,-12))*((-6,-24,18).(-6,-24,18))))
      c) arccos((2,1,-8).(7,2,2)/sqrt(((2,1,-8).(2,1,-8))*((7,2,2).(7,2,2))))
      d) arccos((5,-3,2).(1,3,4)/sqrt(((5,-3,2).(5,-3,2))*((1,3,4).(1,3,4))))

1.15  a) (2,-5).(3,4)/sqrt((3,4).(3,4)) * (3,4)/sqrt((3,4).(3,4))
      b) (4,-5,7).(-3,-2,4)/sqrt((-3,-2,4).(-3,-2,4)) * (-3,-2,4)/sqrt((-3,-2,4).(-3,-2,4))
      c) (2,1,-3).(7,-2,4)/sqrt((2,1,-3).(2,1,-3)) * (2,1,-3)/sqrt((2,1,-3).(2,1,-3))

1.17  arccos((3,0,4).(7,0,1)/sqrt(((3,0,4).(3,0,4))*((7,0,1).(7,0,1))))

1.18  arccos((1,1,0).(1,1,1)/sqrt(((1,1,0).(1,1,0))*((1,1,1).(1,1,1))))