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Summary – 10 Nov 2009 (prepared by David Henderson) Chapter 13. It is OK to use algebra results such as: (a+b)2 = a2 + b2 +2ab but first show by picture that they are true by dissection. In 13.4, if /2 then the areas of the parallelograms formed are not equal to ac or bd -- what is the area? Chapter 20. In 20.1 use Figure 20.1 and relate the extrinsic radius r’ to the intrinsic radius r. In 20.2b, draw a triangle on a sphere with one angle and place the sphere on a piece of paper so that this vertex is on the paper. Now project from the center of the sphere (gnomic projection). O Rc b a C G(B) G(A) To better see the details of this picture, we look separately at four triangles: O O b* a* G(A) b* b a a* S a' b' G(B) c S a' G(A) G(A) b' c' Now use the planar Law of Cosines. G(B) c' G(B)