



 
Hi Carla, I used vectors to solve this problem. First put a coordinate system on 3space with one of the vertices of the base of the pyramid at the origin O and two of the sides of the base lying on the X and Y axes as in the diagram. P has coordinates (6, 0, 0), R has coordinates (0, 6, 0) and S has coordinates (3, 3, 8). The vector cross product OS × OR gives a vector u which is perpendicular to the triangular face SOR. The vector cross product OP × OS gives a vector v which is perpendicular to the triangular face SPO. The angle between u and v is angle between the faces SOR and SPO. Use the vector dot product of u and v to find the angle between them. Penny  


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