All complex numbers can be written as formula_9 (or formula_10), where "a" is called the "real part" of the number, and "b" is called the "imaginary part". We write formula_11 or formula_12 for the real part of a complex number formula_13. So, if formula_14, we write formula_15. Similarly, we write formula_16 or formula_17 for the imaginary part of a complex number formula_13; formula_19, for the same . Every real number is also a complex number; it is a complex number with formula_20.