# Comples Numbers

1. Mar 7, 2010

### EngWiPy

Complex Numbers

Hello,

Why when dealing with complex numbers, as with multiplication, we use the complex conjugate operator?

Regards

Last edited: Mar 7, 2010
2. Mar 7, 2010

### Studiot

So we can do complex 'division' and fractions.

Compare what happens in the following

$$\frac{{a + ib}}{{c + id}}*\frac{{c - id}}{{c - id}}$$

with

$$\frac{{a + ib}}{{c + id}}*\frac{{c + id}}{{c + id}}$$

3. Mar 7, 2010

### EngWiPy

Re: Complex Numbers

I didn't get it. I mean, in telecommunication systems, when we deal with baseband signals, we deal with complex numbers, and all the time we use the complex conjugate operator, but I don't understand why and what it is mean physically.

4. Mar 7, 2010

### Studiot

I did expect you to work my examples out.

Which one contained the conjugate and which one leads to a single complex number result?

If you apply a formula in real analysis say 27*3 you want the simple answer 81, not something more difficult than you started with such as

$${\left( {\sqrt 9 } \right)^2}*{\left( {\sqrt 9 } \right)^2}$$

The same is true of complex numbers.

What does simple multiplication by a conjugate yield by the way ( a real number)?

5. Mar 7, 2010

### HallsofIvy

The product of a complex number and its conjugate has the nice property that is a real number- and for any z other than 0 $z*\overline{z}$ is a positive real number.