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This solving technique is based on avoiding a deadly rectangle.
A deadly rectangle can only exist in an invalid puzzle (multiple solutions).
A Hidden Rectangle is similar to a Unique
Corner.
In the Unique Corner, three of the four cells of the
rectangle are bivalue. In the Unique Side, two of the cells
are bivalue. A Hidden Rectangle has only one bivalue
cell in the rectangle.
The trick here is to focus on the cell diagonally opposed to the bivalue cell. If placing a 5 or 8 in this cell forces a deadly rectangle, that candidate can be eliminated.
Placing a 5 in that cell does not force the two remaining cells to
become 8's. However, placing an 8 in that cell does force the
other two remaining cells to beome 5's. This is because there are
only two 5's in column 3 and in row 8.
Therefore, that cell cannot be an 8.
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