Thread: rearrangement

Hi,
Could you explain me this rearrangement?
Thanks in advance

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

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