# Homework Help: Expressing as complex numbers

1. Mar 14, 2008

### Ry122

How do I express the following in the form a+ib?

$$(\sqrt{-4} - 1) (\sqrt{-9})$$

My attempt:
(2i - 1)(3i)
6i-3i = 3i

Last edited: Mar 14, 2008
2. Mar 14, 2008

### StatusX

Close, but (2i)(3i)=6i2=-6, not 6i.

3. Mar 14, 2008

### Ry122

Can you tell me why the answer -6-+3i has plus or minus 3i?

Also can you tell me the answer to (3+i)^2 - (3+i)^2
The answer I got was 0.
9+6i+i^2-9-6i-i^2=0

Last edited: Mar 14, 2008
4. Mar 14, 2008

### Dick

You have to define what you mean by sqrt(-n) for n>0. You could choose it to be either i*sqrt(n) or -i*sqrt(n). That's your sign ambiguity.

5. Mar 14, 2008

### HallsofIvy

What was that the answer to?? Surely not the (2i-1)(3i) problem you were just talking about!

The answer to a2- a2 is 0 no matter what a is!