Solid Mechanics questions - A new challenge

[dislect]

Solid Mechanics questions - A new challenge :)

Hi all

I got 3 new questions that need to be solved in about 3-4 days as part of a 'challenge' homework. Those who solve correctly will basically get their names read out loud on the next lecture but I do it to understand the material of the course

First Question:

http://up185.siz.co.il/up3/n2i5uwddmczo.jpg

I first tried to write all that I think that has to do with the questions, expressions that I learned and think that I'll use.
Then I thought of first expressing E1111 but then I figured there's no need to.
How do I express E'1111 ? do I use the last formula (transition formula) first?

 

[Mapes]

The solution doesn't involve any sines or cosines (or trigonometry at all). It's hard to say more without giving the "Aha!" away.

 

[dislect]

My thoughts are that I should find E1111 and then E'1111 will be equal to E1111 since the stiffness matrice E does not change from one system to another (?)

 

[Mapes]

Agreed, because the material is isotropic.

 

[dislect]

Thanks!
Now for the more challenging part:

I haven't met any student in my semseter that managed to get any progress in that question yet.

 

[dislect]

help, anyone ?