Problem:
How many different patterns can be made by shading exactly two of the nine squares? Patterns that can be matched by flips and/or turns are not considered different.
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For example, the patterns shown below are not considered different.
Answer Choices:
A.
B.
C.
D.
E.
Solution:
There are . Be systematic. You can begin with the five cases that have one corner square. Then consider the other three cases that do not have a corner square.
Answer: .
The problems on this page are the property of the MAA's American Mathematics Competitions