5. connect co, AO and MN. M, n, O n and o are the midpoint of BC, CD and BD respectively, and AB=BC=CD=DA.

So MN is perpendicular to CO, MM//BD, AO is perpendicular to BD, so MN is perpendicular to AO, so MN is perpendicular to plane AOC.

6. (1) Connect CD 1. If there is CD1/A 1B, then the angles CD 1C 1 are A1b and C/kloc-0.

Tan jiao CD1c1= cc1/c1d1= root number 3, so the angle CD 1C 1=60 degrees.

(2)

7. (1) connects AC and BC, so AC is perpendicular to BC, PA is perpendicular to the plane where the circle lies, so PA is perpendicular to BC, so BC is perpendicular to the plane PAC, so PC is perpendicular to BC, so the angle PCA=45 degrees, so the triangle PAC is an isosceles right triangle, and M is the midpoint of PC, so AM is perpendicular to PC.

(2) connect BM. According to (1), BC is perpendicular to PAC, BC is perpendicular to AM, AM is perpendicular to PC, AM is perpendicular to PBC, so AM belongs to AMB, so AMB is perpendicular to PBC.