Applied Mechanics and Graphic Statics
The centre of gravity of a trapezoidal dam section whose top width is a, bottom width is band the vertical side is a, from its vertical face is

None of these
(a² + ab + c²)/3 (a + c)
(b² + bc + c²)/3 (b + c)
(a² + ab + b²)/3 (a + b)

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Applied Mechanics and Graphic Statics
The angle of friction is:

The ratio of the friction and the normal reaction
The force of friction at which the body is just about to move
The angle between the normal reaction and the resultant of normal reaction and limiting friction
The force of friction when the body is in motion

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