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## 'What's Possible?' printed from http://nrich.maths.org/

*You may be interested in Hollow Squares which offers an alternative way of thinking about the same underlying mathematics.*
Many numbers can be expressed as the difference of two perfect squares. For example, $$20 = 6^2 - 4^2$$ $$21 = 5^2 - 2^2$$

How many of the numbers from $1$ to $20$ can you express as the difference of two perfect squares?

### Here are some questions to consider:

What do you notice about the difference between squares of consecutive numbers?

What about the difference when I square two 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 make?

Can you prove any of your findings?

You may want to take a look at Plus Minus next.