All complex numbers come in two parts: the real and the imaginary ones. That's why we can write all complex numbers as
[tex]a+bi[/tex]
where [tex]a[/tex] is the real coefficient, and [tex]b[/tex] is the imaginary coefficient.
In particular, you can see that, if you set [tex]b=0[/tex], you have the usual real numbers again. For example, choose [tex]a=5[/tex] and [tex]b=0[/tex] to get
[tex]5+0i=5[/tex]
So, real numbers are actually already complex numbers, just like you can think of integers as fraction, where
[tex]5=\dfrac{5}{1}[/tex]
