# Thread: Difficult Proof

1. ## Difficult Proof - Does ANYBODY know how to do this?

Prove that the angle between the diagonal of a cube is not dependent upon the edge length of the cube.

2. using vectors is faster . . . .

3. How about showing some effort!

We want to teach you to fish, not give you a fish.

4. Originally Posted by Jameson
How about showing some effort!

We want to teach you to fish, not give you a fish.

You see, I have tried all I can - I have reached nothing! PLEASE HELP!

5. There are different ways of going about this. What class are you taking and what topic on you on?

6. Originally Posted by yeah:)
Prove that the angle between the diagonal of a cube is not dependent upon the edge length of the cube.
You can try to find the value of this angle for any cube. And then show that it's constant and do not depend of cube's edge length.

7. Originally Posted by yeah:)
You see, I have tried all I can - I have reached nothing! PLEASE HELP!
Then show what you have tried!
Prove that the angle between the diagonal of a cube is not dependent upon the edge length of the cube.
Do you understand that "between" requires two things? The angle between the diagonal and what?

One thing you might try: Set up a coordinate system with one corner of the cube at (0,0,0) and the others at (s, 0, 0), (0, s, 0), (0, 0, s), etc. That's similar to pacman's suggestion that you use vectors.

8. If this is a trig-class (as it's in the Trig Forum Section. . .), they really aren't looking for a "proof", so much as you reasoning out an example. An approach would be to be one side of the cube and place it on the normal x,y plane and go about solving for the diagonals. You would have to use some reasoning (drawing a diagram would help) to show this for diagonals moving in 3 spaces but it can be done (assuming you are not familiar with vectors, as has been assumed here).