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A-72.

Sylvia simplified the expressions on the Expression Comparison Mat shown at right. Some of her work is shown. Are all of her moves “legal”? Explain. 

Expression Comparison Mat Which is greater?. Upper Left has 2 positive x tiles, and 1 negative unit tile. Upper Right has 2 positive x tiles, & 1 positive unit tile. Both the Lower Left and right has 1 positive unit tile. Movement. Step1. Remove 1 negative unit tile and 1 positive unit tile from the left. Step 2. Remove 2 positive x tiles from both the upper left and right. Step 3. Move 1 positive unit tile from lower right to upper right flipping it to negative. Then remove both unit tiles.

Shaded tile is plus 1.  Tile is negative 1.

Go through your list of legal moves and check each move made by Sylvia.

Flipping tiles from the subtraction region to the addition region and changing it from positive to negative is a legal move.

Removing a positive tile from the (-) region and a negative tile from the (+) region is not a legal move, because they do not make a zero pair.

Removing the same amount from both sides is a legal move.

Removing zero pairs is a legal move.