Problem 28. The point Q(5,-2) is the vertex of the angle. We call the given points on the two sides of the angle P = (4,-1) and R = (0,0). The line joining P and Q is given in normal form as (1,1),X = 3, or equivalently x + y = 3. The line joining Q and R is given in normal form as (2,5),X = 0, or equivalently 2x + 5y = 0.
By definition, the interior of the angle consists of all points that are on the same side of PQ as R and also on the same side of QR as P. We calculate: (1,1),R = (1,1),(0,0) = 0 < 3. This shows that the points on the same side of PQ as R are characterized by the inequality (1,1),X < 3, or equivalently x + y < 3.
A similar calculation shows that
(2,5),P = (2,5),(4,-1) = 3 > 0.
This shows
that the points on the same side of QR as
P are characterized by the inequality
(2,5),X > 0, or equivalently
2x + 5y > 0. Therefore, the interior
of the angle is characterized by the following system of inequalities:
x + y < 3 ;
2x + 5y > 0 .
Problem 53. One side of the angle has direction indicator (3,4) - (2,-1) = (1,5). The length of this vector is 261/2 [square root of 26 . . .], so that we obtain the unit direction indicator U = 1/261/2 (1,5). The other side of the angle has direction indicator (4,5) - (2,-1) = (2,6), which has length = 401/2 = 2·101/2. Therefore, the unit direction indicator of this side is V = 1/401/2(2,6), or V = 1/101/2(1,3). To find the angular measure, we first calculate the inner product:
U,V = |
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· |
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(1,5),(1,3) = |
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(1,5),(1,3) = |
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= |
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Problem 69. Let the equation of l be given in normal form as A,X = c, and let the equation of k be A,X = b. (We can use the same vector A in both equations, since the lines are parallel to each other. According the formula in Theorem 23, the distance to l from a point Y is:
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Problem 70. {Not assigned} Let the equations of the lines k and l be as in Problem 69. By the result of that problem, the distance to l from a point on k is |c - b| / ||A||, while the distance to k from a point on l is |b - c| / ||A||. Since the absolute value of any real number is equal to the absolute value of its negative, we conclude that these two numbers are equal.
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Prof. Joel Roberts
School of Mathematics
University of Minnesota
Minneapolis, MN 55455
USA
Office: 351 Vincent Hall
Phone: (612) 625-1076
Dept. FAX: (612) 626-2017
e-mail:
roberts@math.umn.edu
http://www.math.umn.edu/~roberts