6 ms·

Here’s a “Russian” problem: find the largest projection on the plane of a box with sides a, b, c.

btmorex 15y ago
Well, I don't care about my math rep so... I don't get it. You've listed 3 sides of a box. Does that mean that the "fourth" side is part of one of those 3 sides so a = b = c? Or are you asking for the orientation and relative length of the 3 sides such that the fourth side would maximize the area of the box?
- sliverstorm 15y ago
  You don't know how to represent a rectalinear shape with only 3 sides? Man, you really are behind. ... kidding :)
- nandemo 15y ago
  The "3 sides" are the height, width and depth of the box.
- losvedir 15y ago
  I think the question is asking something akin to "You have a box with width a, height b, and length c. What's the biggest shadow you can make?" For example, if you think of the sun straight overhead, then the shadow will have area a*b. But if the sun moves over, then the shadow will grow by inclusion of some of the height of the box.
bbrizzi 15y ago
Thanks to you, I just spent half my morning trying to solve this without getting anywhere. I tried using vectors a, b and c that i project unto the xOy plane to get the sum of three parallelograms. But then the problem is to parametrize these three vectors' positions. The first one can be define in spherical coords as (acos(phi)sin(psi), asin(phi)sin(psi), a*cos(psi)) but then I need to introduce a third parameter for vector b and write both b and c using this system... I'll give it another try this afternoon using matrices. EDIT: Alright, I guess you can write a, b and c as a rotational matrix using Euler angles ( http://en.wikipedia.org/wiki/Euler_angles http://en.wikipedia.org/wiki/Euler_angles ). You can probably also cancel one of the angles (let's say theta) by rotating your xOy plane around which doesn't change the problem. You then just have to calculate S using phi and psi. S = ||a^b||+||b^c||+||c^a|| and solve dS/d(psi) = 0 and dS/d(phi) = 0 However, that's way too much trig for me for today so I'll leave it to someone more courageous than me.

Well, I don't care about my math rep so... I don't get it. You've listed 3 sides of a box. Does that mean that the "fourth" side is part of one of those 3 sides so a = b = c? Or are you asking for the orientation and relative length of the 3 sides such that the fourth side would maximize the area of the box?

sliverstorm 15y ago
You don't know how to represent a rectalinear shape with only 3 sides? Man, you really are behind. ... kidding :)
nandemo 15y ago
The "3 sides" are the height, width and depth of the box.
losvedir 15y ago
I think the question is asking something akin to "You have a box with width a, height b, and length c. What's the biggest shadow you can make?" For example, if you think of the sun straight overhead, then the shadow will have area a*b. But if the sun moves over, then the shadow will grow by inclusion of some of the height of the box.

bbrizzi 15y ago

Thanks to you, I just spent half my morning trying to solve this without getting anywhere. I tried using vectors a, b and c that i project unto the xOy plane to get the sum of three parallelograms. But then the problem is to parametrize these three vectors' positions. The first one can be define in spherical coords as (acos(phi)sin(psi), asin(phi)sin(psi), a*cos(psi)) but then I need to introduce a third parameter for vector b and write both b and c using this system... I'll give it another try this afternoon using matrices. EDIT: Alright, I guess you can write a, b and c as a rotational matrix using Euler angles ( http://en.wikipedia.org/wiki/Euler_angles http://en.wikipedia.org/wiki/Euler_angles ). You can probably also cancel one of the angles (let's say theta) by rotating your xOy plane around which doesn't change the problem. You then just have to calculate S using phi and psi. S = ||a^b||+||b^c||+||c^a|| and solve dS/d(psi) = 0 and dS/d(phi) = 0 However, that's way too much trig for me for today so I'll leave it to someone more courageous than me.