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This is a graph with two perpendicular axes, labelled x and y. Any point in the plane can be
specified by two numbers, its distance along the x-axis, and its distance along the y-axis, as shown.

Sometimes it is useful to think of the point (a,b) as a complex number **a+bi**, where **i** stands for
the square root of minus one.

Of course, the square root of minus one does not really exist. We call it an imaginary number.
And then we call **a** the real part of **a+bi** and **b** the imaginary part. Some times we use the names real
and imaginary axes for the **x** and **y** axes respectively.

We can do arithmetic with complex numbers, as long as we remember that **i*i = -1**

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