[Tex/LaTex] Left justify equation in proof but keep = alignment

alignmath-modevertical alignment

\begin{proof}
Let $g_1, g_2 \in G$. Then $\theta(g_1) = Ng_1$ and $\theta(g_2) = Ng_2$. \\
\begin{equation*}
\begin{split}
\text{ Now, } \theta(g_1g_2) &= Ng_1g_2 \\
& = Ng_1 Ng_2 \\
& = \theta(g_1) \theta(g_2)\\
\end{split}
\end{equation*}
$\therefore \theta$ is a group homomorphism. $\theta$ is clearly onto.
\end{proof}

Is there a way to left justify the first line of the equation, but then keep the = signs aligned in the subsequent lines? I've tried many combinations of \begin{align} etc with no luck. (It appears that the \begin{equation} favors center alignment.)

Best Answer

You can do it with aligned. I also provide what I firmly believe is a better version.

\documentclass{article}
\usepackage{amsmath,amssymb,amsthm}

\begin{document}

\begin{proof}
Let $g_1, g_2 \in G$. Then $\theta(g_1) = Ng_1$ and $\theta(g_2) = Ng_2$. \\
$\begin{aligned}
\text{Now, } \theta(g_1g_2) &= Ng_1g_2 \\
& = Ng_1 Ng_2 \\
& = \theta(g_1) \theta(g_2)\\
\end{aligned}$\\
$\therefore \theta$ is a group homomorphism. $\theta$ is clearly onto.
\end{proof}

\begin{proof}
Let $g_1, g_2 \in G$. Then $\theta(g_1) = Ng_1$ and $\theta(g_2) = Ng_2$. Now
\begin{align*}
\theta(g_1g_2) &= Ng_1g_2 \\
& = Ng_1 Ng_2 \\
& = \theta(g_1) \theta(g_2)
\end{align*}
Therefore $\theta$ is a group homomorphism. $\theta$ is clearly onto.
\end{proof}

\end{document}

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