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How many of the numbers from $1$ to $30$ can you express as the difference of two perfect squares?
What do you notice about the difference between squares of consecutive numbers?
What about the difference between the squares of numbers which differ by $2$? By $3$? By $4$...?
When is the difference between two square numbers odd?
And when is it even?
What do you notice about the numbers you CANNOT express as the difference of two perfect squares?