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**phgao** How come when you work out the answer (d) this way it is different? Help!

: First drop the perpendicular from O - the top to the front line AB. From this we work out angleOBA is 72.45 degrees.

: Say A is the front right corner of pyrimid, then drop a perpendicular to line OB where O is the upper vertex and B is the front left had corner.

: Look at this right angled triangle; we already know OBA - 72.45, and we know AB - 10cm. So we can work out the line say Q from A perpendicular to the point on OB say P.

:So Q = 9.5346cm

:Then join points A and C (back right corner), join C to P, and join P to A. We have a triangle. Let angle APC = F which is the angle we want.

:So cosine rule:

c^2 = a^2 + b^2 - 2ab(cosF)

cosF = a^2+b^2-c^2/2ab

cosF = 2*9.5346^2-200/2*9.5346^2

...

F = 118.47 degrees?! Where is the mistake if there is one?