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Geometric Constructions for Love and Money Kevin Mirus Madison Area Technical College Wisconsin Mathematics Council 36th Annual Green Lake Conference Friday May 7, 2004 [email protected] http://www.matcmadison.edu/is/as/math/kmirus/ Page 1 of 29 Some types of geometric construction (Ref. 1): 1. Euclidian (Ruler and Compass) constructions use a ruler with no markings and a compass which can only be used to draw a circle with a given center point through another given point. Angles cannot be trisected under these restrictions. 2. Mohr – Mascheroni (Compass Only) constructions use a compass alone; anything that can be constructed with a ruler and compass can be constructed with a compass alone (!). 3. Ruler constructions use a ruler alone; anything that can be constructed with a ruler and compass can also be constructed with just a ruler and four given vertices of a trapezoid (!). 4. Ruler and Dividers constructions use a ruler and a divider to carry distance. This allows some constructions that a ruler and compass don’t. 5. Poncelet-Steiner and Double Ruler constructions use either a ruler and a circle with a known center or a ruler with two parallel edges. All ruler and compass constructions can be constructed under these circumstances. 6. Ruler and Rusty Compass constructions use a ruler and a compass of fixed opening; anything that can be constructed with a ruler and compass can be constructed with a compass alone (!). 7. Stick constructions use sticks of equal length that can be laid on top of each other. You can do more constructions with sticks than with a ruler and compass. 8. Marked Ruler constructions use a ruler with two marks on its edge. You can do more constructions with a marked ruler than with a ruler and compass. 9. Paperfolding constructions use folds in paper. Any constructions of plane Euclidian geometry can be executed by means of creases. 10. Constructions using other instruments like a “tomahawk”, carpenter’s squares, strings, etc. allow for constructions not possible using a ruler and compass. 11. Approximate constructions are useful when close is good enough… Page 2 of 29 Problem: Construct a line segment congruent to a given segment. Step 2: Use a compass to measure the distance AB, then draw an arc centered at C with radius AB that intersects l at point D. Comments: This is not a strict Euclidian construction, since the compass was used to transfer a distance. This construction can also be used to construct the sum and difference of two given segments. Given: Line segment AB Step 1: Choose any point C on line l. Problem: Construct a line segment congruent to a given segment by folding paper. Given: A line segment. Step 1: Fold the given line segment onto the other line. Step 2: Crease the paper at either endpoint of the given segment. The intersection of the crease and line are congruent. Comments: Problem: Construct a circle whose radius is equal to the length of a given segment. Given: Line segment PQ , point R Step 1: Draw ray PR . Continued… Page 3 of 29 Step 2: Draw an arc centered at P through Q that intersects PR . Label the intersection point A. Step 3: Draw arcs centered at points Q and A through point P. Label their intersection X. Step 5: Draw an arc centered at P through R that intersects PQ . Label the intersection point B. Step 4: Draw ray QX . Step 6: Draw arcs centered at points R and B through point P. Label their intersection Y. Step 7: Draw ray RY . Label the intersection of RY and QX point S. Page 4 of 29 Step 7: Draw the circle centered at R through S. RS = PQ, since PQRS is a parallelogram. Thus, any radius of this circle is equal to PQ. Comments: This is a strict Euclidian construction for congruent segments. Its importance is to show that a Euclidian compass is equivalent to a modern compass (i.e., it's okay to use a compass to remember distances). Ref. 1. Problem: Construct an angle congruent to a given angle. Given: Angle BAC Step 1: Choose any point D on line l. Continued… Page 5 of 29 Step 2: Draw an arc of any radius centered at A. Label the points of intersection P and Q. Create the same radius arc centered at D. Label the point of intersection E. Step 3: Draw an arc centered at P through Q. Draw an arc centered at E with the same radius. Label the intersection point F. Step 4: Draw ray DF . BEDF is congruent to BAC Comments: This construction can also be used to construct the sum and difference of angles. A good exercise is to construct a 105 angle using ruler and compass… Page 6 of 29 Problem: Construct an angle congruent to a given angle by folding paper. Given: An angle. Step 1: Fold the given angle onto the other line. Step 2: Crease the paper on the other edge of the angle. The crease and line form a congruent angle. Comments: Problem: Construct the perpendicular bisector of a given line segment. Step 1: Draw arcs centered Step 2: Draw segment PQ . on A and B that have the Label its intersection with same radius greater than the AB as point M. distance to the midpoint. Label the intersections P and Q. Comments: Constructing the perpendicular bisector gives you the midpoint for free! Given: Line segment AB Problem: Construct a perpendicular bisector by folding paper. Given: A segment. Step 1: Create the crease between the endpoints. Comments: Page 7 of 29 Step 2: Fold one endpoint onto the other. This crease is the perpendicular bisector. Problem: Construct the bisector of a given angle. Given: Angle BAC Step 1: Draw an arc of any radius centered at A. Label the points of intersection P and Q. Step 2: Draw arcs of equal radius from P and Q. The line from A through their intersection bisects BAC . Comments: Problem: Construct the bisector of a given angle by folding paper. Given: An angle. Step 1: Fold one edge of the angle onto the other edge. Comments: Cool, huh? Page 8 of 29 Step 2: The crease bisects the angle. Problem: Trisect a given angle using Pappus' Theorem / Method of Archimedes. Given: Angle ABC Steps: Construct perpendiculars and parallels through A. Draw an arc centered at A that passes through B in the interior of the circle. Draw ray BS such that MS = RM = AB. Comments: This is a marked ruler construction… Problem: Trisect a given angle using Archimedes' method. Given: Angle ABC Steps: Draw a circle centered at B that passes through A and C. Draw line AQ such that PQ = BC = AB = PB. Comments: This is another marked ruler construction… Page 9 of 29 Problem: Trisect a given angle by folding paper. Given: Angle PQR . Call PQ line l, and QR line m. Step 2: Construct the perpendicular to line m through point M by folding line m onto itself such that the crease contains point M. Call the crease that is made line b. Step 1: Locate the midpoint M of PQ by folding line l onto itself so that point P lands on top of point Q. Call the crease that is made line a. Step 3: Construct the perpendicular to line b through point M by folding line b onto itself such that the crease contains point M. Call the crease that is made line c. Notice that line c is parallel to line m. Continued… Page 10 of 29 Step 4: Fold the paper so that point P lands on Step 5: Fold the paper so that the crease line b and point Q lands on line c. This is tricky contains points Q and Q’. Call the crease line e. and will take some sliding around to get right. Line e trisects PQR . On the diagram above, point P landed on top of line b at point P’, while point Q landed on line c at point Q’. Call the crease that is made line d. Comments: You can do more with paperfolding than with a ruler and compass! Problem: Trisect a given angle using the Quadratix of Hippias. Given / Step 1: The quadratix of Hippias is a y curve defined by the equation x y cot . 2 Draw the curve and lay the angle to be trisected with its vertex at the origin and one edge on the x axis. Continued… Step 2: Mark the point where the angle intersects the quadratix curve. Construct a line parallel to the x axis through this point. Page 11 of 29 Step 3: Take the segment on the y axis from the Step 4: Draw the line connecting the origin and origin to the intersection of the horizontal line the point of intersection between the previous and divide it into three pieces using a geometric horizontal line and the quadratix. This line construction. Construct a line parallel to x axis trisects the original angle. through the bottom third. Comments: The quadratix curve can also be constructed “dynamically”… Page 12 of 29 Problem: Trisect a given angle using a tomahawk (hatchet). Comments: Problem: Divide a given angle into any odd number of equal angles using hinged tomahawks. Comments: s Page 13 of 29 Problem: Construct a line perpendicular to a given line and containing a point not on the given line. Comments: Problem: Construct a line perpendicular to a given line and containing a point not on the given line by folding paper. Fold the line onto itself and form a crease that contains the point. Comments: Problem: Construct a line perpendicular to a given line and containing a point on the given line. Comments: Page 14 of 29 Problem: Construct a line perpendicular to a given line and containing a point on the given line by folding paper. Fold the line onto itself and form a crease that contains the point. Comments: s Problem: Construct an equilateral triangle. Comments: Problem: Construct an equilateral triangle by folding paper. Make a square, place a crease up the middle, then fold a vertex of the square onto the middle crease. Comments: Problem: Construct an equilateral using the vesica pisces. Comments: Things are always a little more interesting with subversion and religion mixed in… Page 15 of 29 Problem: Construct a line parallel to a given line through a point not on the given line. Comments: s Problem: Construct a line parallel to a given line through a point not on the given line. Comments: s Problem: Construct a line parallel to a given line through a point not on the given line using the “draftsman's cheat”. Comments: Page 16 of 29 Problem: Construct a line parallel to a given line through a point not on the given line by paper folding. Construct two perpendiculars… Comments: Problem: Partition a given line segment into a given number of congruent segments. Comments: This is handy in carpentry for laying out stairs… Problem: Locate the center of a given circle. Comments: Page 17 of 29 Problem: Construct the tangent line to a circle at a given point on the circle. Draw a radius to the point, then construct a perpendicular to the radius. Comments: Problem: Construct the circumscribed circle of a given triangle. Comments: Problem: Construct the inscribed circle of a given triangle. Comments: Problem: Construct the "center of gravity" of a given triangle. Comments: Page 18 of 29 Problem: Construct the two tangents to a given circle through a given point outside the circle. Comments: Problem: Construct a circle given its diameter using a framing square. Use the fact that an angle inscribed across a diameter is 90. Comments: This works better than a nail, string, and pencil, I’m told. Problem: Construct a square. Comments: You can now construct an octagon, 16-gon, etc. Page 19 of 29 Problem: Construct a regular pentagon. Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Page 20 of 29 Step 7 Comments: s Problem: Construct a regular hexagon. Comments: This forms the basis for many a church window. Extending the pattern makes a pretty tiling. Problem: Construct a regular hexagon using two vesica pisces. Comments: Page 21 of 29 Problem: Construct a triangle congruent to a given triangle. Comments: Problem: Construct a regular pentagon using a framing square. Comments: Page 22 of 29 Problem: Construct an approximation to any regular polygon. Comments: Problem: Construct a wedge of a 14-gon with seven sticks. Comments: Page 23 of 29 Problem: Construct a Golden Rectangle. Comments: This is very useful in designing furniture with nice proportions. Page 24 of 29 Other Topics: Tangent Circles Page 25 of 29 Page 26 of 29 Other Topics: Spirals Page 27 of 29 Page 28 of 29 References 1. Isaacs, I. Martin, Geometry for College Students, Brooks/Cole Thomson Learning, 0-534-35179-4, 2001. 2. Posamentier, Alfred S. and William Wernick, Advanced Geometric Constructions, Dale Seymour Publications, 0-86651-429-5, 1988. 3. Martin, George E., Geometric Constructions, Springer, 0-387-98276-0, 1998. 4. Sykes, Mabel, Source Book of Problems for Geometry, Dale Seymour Publications, 0-86651-795-2, 1912. 5. "Four Ways to Construct a Golden Rectangle," Fine Woodworking, Taunton Press, February 2004, No. 168, p. 50. 6. Lundy, Miranda, Sacred Geometry, Wooden Books, 0-965-20578-9, 1998. 7. Bedford, John R., Graphic Engineering Geometry, Gulf Publishing Company, 087201-325-1, 1974. 8. "Details on Involute Gear Profiles," How Stuff Works web site (has a great animation of gears meshing), http://science.howstuffworks.com/gear7.htm 9. "Origami and Geometric Constructions," http://www.merrimack.edu/~thull/omfiles/geoconst.html 10. Machinery's Handbook (any edition) 11. "Logarithmic Spiral," mathworld website, http://mathworld.wolfram.com/LogarithmicSpiral.html 12. Wingeom for Windows, website with freeware for geometric constructions (used to create all diagrams in this packet) http://math.exeter.edu/rparris/wingeom.html 13. “Tangent Circles,” a web site: http://whistleralley.com/tangents/tangents.htm 14. “Constructing the Heptadecagon,” a web site: http://www.mathpages.com/home/kmath487.htm Page 29 of 29