# Maths Help - 3D Trig (1 Viewer)

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#### Andy005

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

Just making sure the diagram is correct.

#### 1729

##### Active Member
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

##### Member
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

##### Well-Known Member
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.

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• 1729