I am using flalign since I want text flush with the left explaning
each lines ('from (x)', 'simplify') etc. Some of the lines - the more
important ones - i want numbered and some I do not. I understand I can
use flalign and then \nonumber after each line I do not want numbered.
I also understand I can write a single equation with the equation
environment, label it with my own reference, and use \ref elsewhere to
refer to the number Latex gave it. However how can I label chosen
lines that have been numbered within one flalign environment? Do I
have to keep switching environments?
Here is the code I have. I want the last two lines each with their
separate label so I can refer to them elsewhere. I could do it by
ending the flalign before these last two lines, then write 'using (x)'
and 'similarly' above two separate equation environments that I label,
but I am wondering if there is a neater way whereby I can retain the
text on the same line as the equations, and all in one environment.
\begin{flalign}
& & \text{Let }z & = \cos{\theta}+i\sin{\theta} &&
\nonumber \\
&\Rightarrow & \frac{1}{z} & = \frac{1}{\cos{\theta}+i\sin{\theta}}
\nonumber \\
&\text{Simplify} & & = \frac{1}{\cos{\theta}+i\sin{\theta}}
\times \frac{\cos{\theta}-i\sin{\theta}}{\cos{\theta}-i\sin{\theta}}
\nonumber \\
& & & = \frac{\cos{\theta}-i\sin{\theta}}{\cos^2{\theta}+
\sin^2{\theta}} \nonumber \\
& & & = \cos{\theta}-i\sin{\theta}\nonumber \\
&\Rightarrow &z+\frac{1}{z}& = 2\cos{\theta} \nonumber \\
&\text{and} &z-\frac{1}{z}& = 2i\sin{\theta} \nonumber \\
&\text{Using (\ref{moivre})} &z^n + \frac{1}{z^n}&\equiv 2\cos{n
\theta} \\
&\text{Similarly} &z^n - \frac{1}{z^n} &\equiv 2i\sin{n\theta}
\end{flalign}
Cheers!