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-   -   Finding the dimensions of a rotated rectangle inside another rectangle. (http://www.physicsforums.com/showthread.php?t=311203)

galneweinhaw May2-09 12:05 AM

Finding the dimensions of a rotated rectangle inside another rectangle.
 
1. The problem statement, all variables and given/known data
If I have a rectangle rotated at a known angle with respect to a rectangle of known dimensions that inscribes it, how can I find the dimensions of the inscribed/inner rectangle?

http://bp3.blogger.com/_4Z2DKqKRYUc/Rnz_BgODzFI/AAAAAAAAAIw/uj_cVfPI8D4/s1600-h/Img_6-23-07_Blog.jpg

http://bp3.blogger.com/_4Z2DKqKRYUc/...23-07_Blog.jpg

If the image above is my example, I know the dimensions of ABCD and I know all the angles, such as < BPQ.

2. Relevant equations
Trig/Pythagorous...

3. The attempt at a solution
I'll post if I come up with anything that looks like it's gettign anywhere =P

Thanks for the help... let's see how my first ever post is received =)

diazona May2-09 12:36 AM

Re: Finding the dimensions of a rotated rectangle inside another rectangle.
 
Surely you must have tried something?

Hint: can you find the four triangles in the figure? From there, you have the trig formulas to calculate the lengths of the sides you need...

galneweinhaw May2-09 12:46 AM

Re: Finding the dimensions of a rotated rectangle inside another rectangle.
 
oh, I've been trying for a couple hours. But I haven't really made it anywhere =(

Mark44 May2-09 12:54 AM

Re: Finding the dimensions of a rotated rectangle inside another rectangle.
 
So show us what you've tried.

galneweinhaw May2-09 02:04 AM

Re: Finding the dimensions of a rotated rectangle inside another rectangle.
 
ok... I think I have something that should be able to go somewhere...

Here's a relabelled image:
http://img7.imageshack.us/img7/764/rectb.jpg

ɵ, X, and Y are known, trying to find h and w.

y1, y2, x1, x2, w, and h are the unknowns (6)

I can get seven equations:

w2 = x22+y12

h2 = x12+y22

Y = y1 + y2

X = x1 + x2

y1 = x2 tanɵ

x1 = y2 tanɵ

XY = x2y1 + x1y2 + hw (areas)

galneweinhaw May2-09 02:24 AM

Re: Finding the dimensions of a rotated rectangle inside another rectangle.
 
eliminating x1,x2,y1,y2 I get...

XY = [tex]\frac{(w^2+h^2)tan\theta}{1+tan^2\theta} + hw[/tex]

X = [tex]\frac{htan\theta + w}{\sqrt{1+tan^2\theta}}[/tex]

Y = [tex]\frac{wtan\theta + h}{\sqrt{1+tan^2\theta}}[/tex]

edit:
sub some trig identities

XY = [tex](w^2+h^2)sin\theta cos\theta + hw[/tex]

X = [tex](htan\theta + w)cos\theta[/tex]

Y = [tex](wtan\theta + h)cos\theta[/tex]

galneweinhaw May2-09 10:04 AM

Re: Finding the dimensions of a rotated rectangle inside another rectangle.
 
Further simplifying...

[tex]X = hsin\theta + wcos\theta[/tex]

[tex]Y = wsin\theta + hcos\theta[/tex]

LOL.... I could have pulled that directly off the diagram! well, at tleast I know my algebra is sound =P

igtech May10-11 04:25 PM

Re: Finding the dimensions of a rotated rectangle inside another rectangle.
 
But with this, you find X and Y that it is supposed you already knew, what about finding h and w , huh??

Tommaso_Russo Jun22-11 08:18 AM

Re: Finding the dimensions of a rotated rectangle inside another rectangle.
 
Hi,

I've posted general solutions for all these kinds of problems in a new thread:
http://www.physicsforums.com/showthread.php?t=508715

sorry for the delay, it was not easy.

--
TRu-TS
Buon vento e cieli sereni


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