Maths Help - 3D Trig (1 Viewer)

[Andy005]

https://drive.google.com/open?id=0B_RlXPQTW1cRUGtuc3JJTEVPTU0

I need help with question b), I've tried many variations but I couldn't get the correct answer but I'm probably overthinking it, (i just need to know the angle).

Thanks for the help.

 

[1729]

https://drive.google.com/open?id=0B_RlXPQTW1cRUGtuc3JJTEVPTU0

I need help with question b), I've tried many variations but I couldn't get the correct answer but I'm probably overthinking it, (i just need to know the angle).

Thanks for the help.

 

[Andy005]

So P would be the midpoint of the slanted edge OB and joined to A?

Just making sure the diagram is correct.

 

[1729]

So P would be the midpoint of the slanted edge OB and joined to A?

Just making sure the diagram is correct.

P belongs to the slanted edge OB but it's not the midpoint. AP is just perpendicular to OB. If the triangle was equilateral or if AB = OA then it would be a perpendicular bisector but it isn't.

 

[Andy005]

Can you show me how you would do it?

I ended up with really ugly equations and got an answer of cos114˚44', now I'm really not sure on what to do.

Thanks

 

[Drongoski]

OK - sorry don't know how to upload diagram; so stoopid!

Following 1729: let square base be labelled anticlockwise ABCD (with side AB nearest to you) and the apex E. Now consider the 2 adjacent faces: triangles AEB and AED. Let P be the common foot of the perpendiculars from B to AE and from D to AE. Then consider the triangle PBD. The required angle between the 2 faces is angle DPB of the isosceles triangle DPB.





That's it.