# Imaginary numbers concept help

1. Jan 2, 2012

### rollcast

I've been learning about imaginary numbers and while I understand the concept of them I have tried a few examples with them and I don't get some of the answers.

why can you not take

xi = 90i

and multiply it by i

x*i*i = 90*i*i
x*-1 = 90*-1
-x=-90
x=90

Thanks
AL

2. Jan 2, 2012

### gb7nash

You can. All of your steps are valid. Why do you think you can't do this? (as a side note, you could simply just divide both sides by i)

3. Jan 2, 2012

### rollcast

The book gave xi = 90i as the answer to the problem?

4. Jan 2, 2012

### AlephZero

Possibly we are interpreting your "xi = 90i" wrongly, because that is only an approximate "copy" of the notation in the book.

Can you scan the page or photograph it and attach the image, so we can see exactly what the book says?

5. Jan 2, 2012

### rollcast

Its only a typed additional section that our maths teacher gave us to add to our text book as it didn't cover imaginary and complex numbers.

What I typed, xi = 90i, is exactly how it is displayed on the sheet?

6. Jan 3, 2012

### Mentallic

The answer would never be xi = ... where x is the value you were trying to find and i is the imaginary number. Either it's meant to be xi where i is subscripted or it's simply a typo.

Think about it, would the answer book ever intentionally say 2x = 10?

7. Jan 3, 2012

### rollcast

I just compared my book with a friends and the rest of the solution is on the back of the page but its blank on mine.

8. Jan 3, 2012

### Mentallic

Well, this is the homework help section, so if you need a solution to a problem you can go about it by asking us help on the parts of the problem you're unsure of.