State the Problem  
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The figure shows a triangular plate P Q R S before and after deformation. The base of the triangle is Q R S and it is fixed to the ceiling. The length of the segment Q R is 900 millimeters and the length of the segment R S is 300 millimeters. The apex of the triangular plate is P. The angle at Q is 30 degrees and the angle at S is 60 degrees. After deformation, the apex P, extends downwards by 1 millimeter.

A thin triangular plate is uniformly deformed. Determine the shear strain at P after point P has been displaced 1 mm downward.