Discussion about math, puzzles, games and fun.   Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫  π  -¹ ² ³ °

You are not logged in.

## #1 2013-01-12 17:26:12

rajinikanth0602
Member
Registered: 2012-06-30
Posts: 16

### another challenging problem for all

can any one say the angle between the planes in a pentagon ball???????

Offline

## #2 2013-01-12 19:15:17

bob bundy
Moderator
Registered: 2010-06-20
Posts: 7,925

### Re: another challenging problem for all

hi rajinikanth0602

Yes, I expect someone can.  But shouldn't you respond to

http://www.mathisfunforum.com/viewtopic.php?id=18761

first?

Bob

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

Offline

## #3 2013-01-13 18:44:20

Agnishom
Real Member
From: Riemann Sphere
Registered: 2011-01-29
Posts: 24,722
Website

### Re: another challenging problem for all

According to wikipedia, and what I understood, such a solid is a dodecahedron.
And the requied angle is 116.57°.

I shall be waiting eagerly for someone to explain why

'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'
'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'
I'm not crazy, my mother had me tested.

Offline

## #4 2013-01-13 20:53:35

bob bundy
Moderator
Registered: 2010-06-20
Posts: 7,925

### Re: another challenging problem for all

hi Agnishom,

I've done a few of these over the last two years.  The method uses vector geometry and the cosine rule.  See

http://www.mathisfunforum.com/viewtopic.php?id=16061

Post 30.

I think Wiki also gives the co-ordinates of the points in 3D so you can pick two adjacent faces, get the critical lines and apply the method.

I've got a post to make for MIF, so It'll be a while before I can put all the figures to this, so have a go yourself.

Bob

ps.  I have a vague memory that I have done it in a thread, but I couldn't find it.

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

Offline