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Circles Portfolio. The purpose of this assignment is to investigate several positions of points in three intersecting circles in order to discover a general statement

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Mathematics SL, Portfolio Type 1Circles


The purpose of this assignment is to investigate several positions of points in three intersecting circles in order to discover a general statement. There are three circles: circle C1 has center O and radius r, circle C2 has center P and radius OP, and circle C3 has center A and radius r (the same radio as C3). Point P' is the intersection of C3 with (OP).

All diagrams in this portfolio were created with Geogebra. To give a clear display of the length of each segment the diagrams are presented in the Cartesian coordinate system using point O as origin and (OP) as x-axis.

At first I will try to find a general statement representing the relation of OP’, OP and r in three examples, in which r will remain constant and OP will have three different values.


Diagram  1: r=1, OP=2

The method I will use to find OP’ will include the cosine function used in two triangles I will sketch in the first three diagrams:

...read more.













Table 1: Results from first part of investigation

In the first part of the investigation with the help of the cosine function we find out that in all cases


 are similar triangles, as in each case they share the same angle measurements (AOP=OAP’, OPA =OAP’ and OAP =AP’O) and the corresponding sides are in proportion (image05.png


When observing the results we find that if each number of OP is multiplied by their correspondent OP’ the result is 1, so this pattern can be generalized by:      image01.png


In the following part of the investigation, I will change the roles of r and OP so that OP will remain constant and r will have three different values.

All values of OP’ were found with Geogebra.


  P (0, 2)



   is an equilateral triangle, as      OP=P’A=AO=2


        P’ (0, 4.5)



       P’ (0, 8)

















Table 2: Results from second part of investigation

Verifying if the statement found in the previous part of the investigation is consistent:

Diagram 4:                                Diagram 5:                                 Diagram 6:


=OP’                                 image01.png

=OP’                           image01.png


...read more.


-Example 9: Radius r is a negative number.

-Example 10:  


                                                      Diagram  11: r=9, OP=4.49

Diagram 11 shows that point P’ is undefined. The combination of values of r=9 and OP=4.5 causes the two circles with radius r to form one single circle. This makes it impossible to define the intersection of C3 with (OP), as C3 does not exist.

The general statement OP’=image01.png

is valid only if r

If r

  • (diagram 11), the value of OP’ is undefined.
  • If r and OP are not bigger than 0 the value of OP’ is undefined.

If r = 2 · OP (diagram 7), the center of C3 is situated on the x-axis. 2·OP is the biggest value r can reach. Triangles


  •  cannot be sketched, but the general statement can still be used for finding OP’.

If r

  •  (diagrams 1, 2, 3, 4, 5, 6, 8, 9,10), OP’ can be found through the general formula.

The process of finding the general statement starts already in the first part of the investigation. With help of the cosine function we find out that in all cases


are similar triangles. The demonstration of the corresponding sides being in proportion can be represented by(image05.png

). By simplifying image06.png

we get to the general statement OP’=image01.png


...read more.

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