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 TOPICS This page:   triangles/polygons    Search   Dr. Math See also the Dr. Math FAQ:   geometric formulas and   naming polygons   and polyhedra and   Pythagorean theorem Internet Library:   triangles/polygons HIGH SCHOOL About Math Analysis Algebra    basic algebra    equations/graphs/      translations    linear algebra    linear equations    polynomials Calculus Complex Numbers Calculators/    Computers Definitions Discrete Math    permutations/    combinations Exponents    Logarithms Fibonacci Sequence/   Golden Ratio Fractals Functions Geometry    Euclidean/plane      conic sections/        circles      constructions      coordinate plane      triangles/polygons    higher-dimensional      polyhedra    non-Euclidean    practical geometry    symmetry/tessellations History/Biography Interest Logic Negative Numbers Number Theory Physics/Chemistry Probability Projects Puzzles Sequences/Series Sets Square/Cube Roots Statistics Transcendental   Numbers Trigonometry Browse High School Triangles and Other Polygons Stars indicate particularly interesting answers or good places to begin browsing. Selected answers to common questions:     Area of an irregular shape.     Classifying quadrilaterals.     Heron's formula.     Polygon diagonals.     Pythagorean theorem proofs.     Triangle congruence. Ratio of Areas of Triangle and Parallelogram [7/17/1996] Given a parallelogram in a triangle... compute the area of the triangle divided by the area of the parallelogram. Ratio of Sides and Ratio of Areas [02/11/1999] If the sides of a triangle are in the ratio of 1:7, what is the ratio of their areas? What about for other shapes? Ratios and Geometry [10/29/1998] An airplane flying at 33,000 feet has a visibility of 100 miles. What percent of the total land area to the horizon is visible? Ratios, Geometry, Trigonometry [06/10/1999] A homeschool teacher asks for help with triangles, flagpoles, and circles. Rectangle Needed To Make Cone [9/5/1996] I want to make a lampshade by rolling a piece of paper into a non-closed cone shape... Rectangle to Parallelogram [06/28/2002] As you change a rectangle to a parallelogram, what happens to the area and the perimeter? Regular and Non-regular Polygon Areas [03/10/1999] Given a regular and a non-regular polygon with the same perimeter, prove that the area of the regular polygon will always be greater. A Regular Nonagon [11/2/1995] I want to know if there is such a thing as a regular nonagon, and if not, why can't you get one? Regular Octagon Problem [09/03/2005] Find the length of the sides of a regular octagon if the distance between opposite sides is 20 feet. Regular Pentagon Construction Proof [11/23/2001] What is the proof of the construction of a regular pentagon? Regular vs. Equilateral Polygons [07/24/2003] What is the difference between a regular polygon and an equilateral polygon? Remembering Area Formulas [12/23/2001] Is there was a good way to help me memorize the formulas for areas of different shapes? Rhombus and Square Comparison [01/14/2004] Comparison of the definitions of rhombus and square as a way to answer the questions, 'Is a square a rhombus?' and 'Is a rhombus a square?'. Rhombus vs. Rhomboid [08/27/2002] What is the difference between a rhombus and a rhomboid? Right Angle [04/09/1997] In a rectangle, draw a line from one vertex to a side to an adjacent vertex. Determine what makes the angle formed in this process 90 degrees. Right Angles in Polygons [07/23/1997] Is there a relation between the number of sides in a polygon and the maximum number of right angles? Right Triangle Inscribed in a Parabola [09/20/1999] Show that the point of intersection Q of the axis of the parabola y=x^ 2 and the hypotenuse of right triangle RST (inscribed in the parabola so that R coincides with the vertex of the parabola) is independent of the choice of right triangle. A Right Triangle of Points [01/14/1999] Determine the values of x that would make the points (x,0), (-2,1), and (3,4) the vertices of a right triangle. Right Triangle Proof [11/19/2004] In right triangle ABC, let CD be the altitude to the hypotenuse. If r1,r2,r3 are radii of the incircles of triangles ABC, ADC, and BDC, respectively, prove CD = r1 + r2 + r3. Right Triangles [9/22/1995] How do you figure the angle of a right triangle when you only have the height and width? Roof Rafters [02/03/1997] What is the formula that gives the length of a roof rafter if only the pitch of the roof is known? Round Robin Tournament Schedule [03/31/2000] Is there a systematic way to come up with a schedule for a round robin tournament for up to 32 teams, where each team plays every other team once? SAT Rectangle Area Question [04/23/2004] In rectangle ABCD, Point E is the midpoint of side BC. If the area of quadrilateral ABED is 2/3, what is the area of rectangle ABCD? Scaling a Right Triangle [12/01/2003] Start with a right triangle of known dimensions. Move the hypotenuse by some distance towards the right angle. What are the dimensions of the new triangle? Secant-Tangent Theorem [03/21/2002] I'm trying to prove the secant-tangent theorem. Shortest Triangle Side [08/10/2002] A triangle has a base of length 13, and the other two sides are equal in length. If the lengths of the sides are integers, what is the shortest possible length of a side? Side Length of a 17-gon [03/03/2002] Given a 17-gon inscribed in a circle of radius 1, what is the length of a side to 6 decimals? Side Length of Octagon Inscribed in Square [9/11/1996] I have a piece of square plywood (48" on a side) and I would like to cut an octagon out of the middle. What is the side length of the octagon? Side Lengths of Isosceles Triangle [7/8/1996] Given two isosceles triangles on top of one another... find the unknown side lengths. Sides of a 30-60-90 Triangle [03/13/1998] In a 30-60-90 triangle where the short side is X, why does the hypotenuse equal 2X and the long side equal X * sqrt(3)? Similar Triangles [1/22/1996] For triangle ABC whose vertices are A(6,3),B(1,5),C(-1,4), what are the vertices of a similar triangle whose perimeter is 5 times that of triangle ABC? Similar Triangles and Area [11/17/1998] P is a point on the segment joining midpoints D, E of the sides AB, AC of a triangle ABC. Prove that BPC has twice the area of ADE. Simson Line [04/19/1999] What is the Simson line? Simson Lines [06/07/2001] Show that, given two triangles inscribed in the same circle, for any point P on the circle the two Simson's lines form a fixed angle. Simson/Wallace Line Proof [11/19/2002] From a point P on the circumcircle of the triangle ABC perpendiculars are dropped to the sides AB, BC, CA. Prove that the line joining the feet of the perpendiculars bisects the line joining the orthocentre of triangle ABC and point P. Sine, Co-sine, and Tangent: SOHCAHTOA [03/28/1999] I am having trouble figuring out what to use when solving a triangle problem. Sine of 36 Degrees [11/18/2001] Ptolemy calculated the sine of 36 degrees geometrically using the construction of a regular pentagon. How did he do it? Sixteen-sided Window [08/05/1997] I would like to make a sixteen-sided window for the second floor hallway of my twin girls' playhouse. Spherical Geometry and Triangles [02/09/2004] Is it possible to have a triangle with two 90 degree angles, where the other two legs from the connected 90 degree angles meet to finish the triangle? Where would you find such a triangle? I thought it might work if the triangle is on a sphere, but then the lines aren't straight. The Spider and the Fly [12/23/1999] A spider and a fly are on opposite walls of a rectangular room... Does the spider get the fly? Page: []

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