# Homework Help: Rotating coordinate axes

1. Feb 15, 2012

### bobsmith76

1. The problem statement, all variables and given/known data

17. xy = 2

3. The attempt at a solution

Do you see that step where they do the following:

√2/2 - √2/2 = my answer is 0

and they multiply that to

√2/2 + √2/2 = my answer is √2

So to me the answer is 0 * √2 = 0, but the book shows that that calculation = 2, then they set x over 2 and y over 2, don't understand why.

I don't understand what the book is doing. I understand everything else except that one part.

Last edited: Feb 15, 2012
2. Feb 15, 2012

### tiny-tim

hi bobsmith76!

3. Feb 15, 2012

### bobsmith76

see revision

4. Feb 15, 2012

### tiny-tim

do you mean the LHS of the equation at the beginning of the last line?

then you're ignoring the x' and y'

that LHS multiplies out to 1/2 x'2 - 1/2 y'2, which you can see is the LHS of the next equation

(and the RHS of both equations stays as 2)

5. Feb 15, 2012

### bobsmith76

are you saying they square √2/2 - √2/2? if they do that is still zero. I don't know what you mean by I'm ignoring x'. What x' am i ignoring.

6. Feb 15, 2012

### tiny-tim

they don't have √2/2 - √2/2 !

are we looking at the same question?

all i'm seeing is something similar with x' and y'

7. Feb 15, 2012

### HallsofIvy

They are NOT dealing with $\sqrt{2}/2- \sqrt{2}/2$, they are dealing with $(\sqrt{2}/2)x'- (\sqrt{2}/2)y'$.

8. Feb 15, 2012

### bobsmith76

ok, thanks. I misread what I was reading I thought it was

(√2/2x' - √2/2x')(√2/2y' + √2/2y')