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

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

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

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

20.5	-(1, -2, 3).(1,-2,1)

20.7	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))))

20.8	CrossProduct[(3,-2,1),(1,0,-2)]
	CrossProduct[(4,7,2),(1,-2,1)]
	-(11,-2,-15).(2,2,1)

20.10	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))

20.13	:(

20.14	a),b),d)  :-(

20.15	CrossProduct[(3,7,-2),(2,-6,7)]

20.16	CrossProduct[(2,-3,4),(0,1,0)]
	

20.17	a) arcsin(|(4,3,1).(3,5,-1)|/sqrt(((4,3,1).(4,3,1))*((3,5,-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) CrossProduct[(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}]