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June 23rd, 2010, 12:42 AM  #11 
Newbie Joined: Nov 2009 Posts: 12 Thanks: 0  Re: geometrical question about congruent rectangles
I have created an image which I want to upload, but it doesn't work... is it due to my status or because of an error? 
June 23rd, 2010, 01:18 AM  #12 
Math Team Joined: Apr 2010 Posts: 2,780 Thanks: 361  Re: geometrical question about congruent rectangles
Hello MG85, I maybe have an image that represents your question. Now, in other words, you are to proof that the area where "2 books are on top of each other, so 2 books thick" is larger than the area where "the area is one layer thick, so no overlay" I hope, the english and the idea are correct. Hoempa 
June 24th, 2010, 02:56 AM  #13 
Newbie Joined: Nov 2009 Posts: 12 Thanks: 0  Re: geometrical question about congruent rectangles
To make it more understandable I created a geo file, as I didn't have webspace I am only able to set a link to a photoalbum... http://de.fotoalbum.eu/gezi999/a490791/00000001 it has to be proved that the green area is always bigger than half of the rectangle's area... 
June 24th, 2010, 03:32 AM  #14 
Math Team Joined: Apr 2010 Posts: 2,780 Thanks: 361  Re: geometrical question about congruent rectangles
Do you have an extra assumption that the centres of the 2 rectangles are on top of each other? The question, skipjack found, doesn't say so. Hoempa 
June 24th, 2010, 09:51 AM  #15 
Newbie Joined: Nov 2009 Posts: 12 Thanks: 0  Re: geometrical question about congruent rectangles
No, there is not an assumption like that, but I didn't know how to realize it with dynageo...

June 24th, 2010, 11:45 AM  #16 
Math Team Joined: Apr 2010 Posts: 2,780 Thanks: 361  Re: geometrical question about congruent rectangles
Suggestion: Maybe you could reflect all the area's with "one rectangle" into the green zone and show there is some space left. Hoempa 
June 25th, 2010, 08:51 PM  #17 
Global Moderator Joined: Dec 2006 Posts: 20,471 Thanks: 2038 
Let p = (area of overlap)/(area of rectangle) and r = (length of rectangle)/(width of rectangle). It seems that p can be considerably larger than 1/2, but can be arbitrarily close to 1/2 if r is sufficiently large. If rectangle ABCD (where AB > BC) is placed first, one can place the second rectangle, A'B'C'D' (where A'B' > B'C'), so that D' is very close to D and A' almost lies on AB. In the limiting case, where D and D' coincide and A' lies on AB, I find that p = rē  r?(rē  1), which tends to 1/2 as r tends to infinity. However, the above placement method isn't optimal if r is not large (since p tends to 1 as r tends to 1), and I haven't proved that it's optimal when r is large. Perhaps a better approach would be to consider the general case, using three variables to specify the position of the second rectangle in relation to the first. That would be a bit tedious, but shouldn't be unduly difficult. 
June 26th, 2010, 01:46 AM  #18 
Math Team Joined: Apr 2010 Posts: 2,780 Thanks: 361  Re: geometrical question about congruent rectangles
I have tried something yesterday, on the puzzle. I think it will be tedious, I donīt even know whether itīs possible to proof so. We could use the cosine rule, generally: I donīt know the formula to calculate the surface of a triangle. I do know this: surface=0.5*side*height, but there must have been another. It used the sides of the triangle. Surface=. Too bad I donīt know the formula. Maybe we could use that to define the surfaces of all areas without overlap. Referring to the image, MG85 posted, The area with overlap could be described by for example , , , and then quadrangle IKEG, by splitting in for example and The formula for surface of triangles will be needed to do so, for we have the sides. Again, this method is tedious if possible to proof so Hoempa 
June 26th, 2010, 02:49 AM  #19 
Global Moderator Joined: Dec 2006 Posts: 20,471 Thanks: 2038 
The usual form of the cosine rule is cē = aē + bē  2ab cos C. Heron's formula gives a triangle's area in terms of the lengths of the sides of the triangle, but is unlikely to be useful here.

June 26th, 2010, 03:12 AM  #20  
Math Team Joined: Apr 2010 Posts: 2,780 Thanks: 361  Re: geometrical question about congruent rectangles Quote:
Anyway, it was worth trying, thanks for the reply!  

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