1. ## rearrangement

Hi,
Could you explain me this rearrangement?

2. ## Re: rearrangement

It looks like they've started by expanding the square.

3. ## Re: rearrangement

That's pretty much it: $(a- b)^2= a^2- 2ab+ b^2$. Here a= T and $b= \frac{H}{\nu_s}$. Then, of course, multiply each term by $\frac{g}{2}$.