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# The segments of a polygon

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Introduction

INTERNATIONAL BACCALAUREATE

II. GIMNAZIJA MARIBOR Portfolio mathematics HL

Assignment 1

The segments of a polygon

Author: Luka Dremelj

Candidate number:

Subject: Mathematics HL

Teacher: Barbara Pećanac

Date written: 25/5/2009

Introduction

First Mathematics HL Portfolio is about investigating the segments of a polygon, using graphical methods and analytical proofs. The task is to find conjecture between ratio of the sides and the ratio of the areas, first of triangles developing it to general conjecture of polygons.

1. In an equilateral triangle ABC, a line segment is drawn from each vertex to a point on the opposite side so that the segment divides the side in the ratio 1:2, crating another equilateral triangle DEF.

(a) What is the ratio of the areas of the two equilateral triangles? To answer this question,

(i) create the above diagram  with your geometry package. (ii) measure the lengths of the sides of the two equilateral triangles = = = 3 units = =  1.13 units

(iii) find the areas of the two equilateral triangles and the ratio between them.

Formula for area of triangle:  (u=unit) I got the same results for the areas, as before calculated with program.               (see the picture above) (b)

Middle

= , = y= = = x= = = p= = =  To get the relationship between the ratios of the sides and the ratio of the areas I have to first get relationship between and . If I look on the first triangle above, I see that , so I have to calculate p, y and x to get .

Triangles and are similar (if two angles of one triangle are same to two angel in second triangle, they are similar). So I can get the relationship of sides of triangles.   To calculate p I have to use cosine’s rule: Now I go again back to relations of sides in similar triangles to calculate x.    now I enter x that I get above into the equation for y and get: And now we have all components to calculate . We enter them in the equation(I have already wrote it down above) :    since I get:  So I prove the conjecture that I get from program in task c.

2. Does this conjecture hold for non-equilateral triangles? Explain. To easily understand this, I draw a non-equilateral triangle which sides are split in ratio 1:2.

Conclusion :   I have all components of so I can calculate it by formula  So I get the general formula for all regular polygons:  (s is number of sides)

To prove this formula I can put in data for my pentagon, which I get with program and compare ratio that I get with my formula and ratio that I get from results of program. and results from my formula Conclusion

Through four tasks I found the conjecture between the ratios of the sides and the ratio of the areas for all regular polygons. I found also some similarities in relationship between areas and sides in non-equilateral triangles, but as I didn’t find the analytical proof, I can’t claim that it holds for all of them.                        I find this task as very interesting and instructively because I learned a lot of new methods (graphical) how to solve problems using advanced computer programs (I also learned how to use these programs as well).  Furthermore I also refreshed my knowledge about geometry and came to some useful findings.

Software used:

GeoGebra(http://www.geogebra.org/cms/)

MATLAB (http://www.mathworks.com/)

LUKA DREMELJ IB 2008/2010

This student written piece of work is one of many that can be found in our International Baccalaureate Maths section.

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