6.2     AB: (x-2)/0 = (y-1)/3 = (z-3)/(-8)
        AC: (x-2)/-2 = (y-1)/3 = (z-3)/(-8)
        BC: (x-2)/-2 = (y-4)/0 = (z+5)/0

6.3     CrossProduct[{3,1,-1},{2,1,2}]
        x/3 = (y-4)/(-8) = z/1

6.4     CrossProduct[{1,0,3},{2,1,2}]
        3x-2y-2z-5=0

6.5     a) |1*1+2*2+2*1-10|/sqrt(1+4+4)
        b) |-1*2+2*8+2*5+1|/sqrt(4+4+1)

6.6     a) CrossProduct[{-3,1,4},{2,1,2}]
        sqrt(4+14^2+25)/sqrt(4+1+4)
        b) CrossProduct[{-5,8,4},{-2,3,0}]
        sqrt(144+64+1)/sqrt(4+9+0)

6.7     a) arccos(|{-2,4,-1}.{7,3,-2}|/sqrt((4+16+1)*(49+9+4)))
        b) arccos(|{0,1,0}.{0,3,-3}|/sqrt((0+1+0)*(0+9+9)))
        c) arccos(|{1,2,3}.{2,4,6}|/sqrt((1+4+9)*(4+16+36)))
        d) arccos(|{1,-2,3}.{-1,2,-3}|/sqrt((1+4+9)*(1+4+9)))

6.8     a) arccos(|{2,1,4}.{3,-2,1}|/sqrt((4+1+16)*(9+4+1)))
           solve x-1 = 2(y-7), -2(x-6)=3(y+1), 4(y-7) = z-5
        b) arccos(|{2,1,4}.{2,1,4}|/sqrt((4+1+16)*(4+1+16)))
        c) arccos(|{1,3,0}.{1,3,0}|/sqrt((1+9+0)*(1+9+0)))
           solve 3(x-1) = y+1, 3x=y-2, z=0
        d) arccos(|{1,3,0}.{3,0,1}|/sqrt((1+9+0)*(9+0+1)))
           solve 3(x-1)=y+1, x-5= 3(z-2), y=-1, z=0

6.9     a) arcsin(|{4,3,1}.{3,5,-1}|/sqrt((16+9+1)*(9+25+1)))
           solve x-12 = 4(z-1), y-9=3(z-1), 3x+5y-z-2=0
        b) arcsin(|{2,4,3}.{3,-3,2}|/sqrt((4+16+9)*(9+9+4)))
           3*(-1)-3*3+2*0-5
        c) arcsin(|{-3,1,2}.{2,-4,5}|/sqrt((9+1+2)*(2+16+25)))
           2*1-4*3+5*0+10
        d) CrossProduct[{3,5,-7},{2,-1,1}]
           arcsin(|{-2,-17,-13}.{5,0,-1}|/sqrt((4+289+169)*(25+0+1)))
           solve 3x+5y-7z+16=0, 2x-y+z-6=0, 5x-z-4=0