Sudoku Tutorial
Hidden Rectangle
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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