# Which environment should I use to align my equations to the left?

I would like to know the "best" environment to align equations to the left. I already use fleqn at the beginning of my document and \begin{aligned} ... \end{aligned} when I write equations in the "enumerate" environment. I would like to align my text like this,

but I don't know what is the best environment to do that and how to do it. Also, sometimes, I get weird indentations, at the beginning of lines depending on the environment and I sometimes have my text left aligned while right aligned (see third block of equation) and I was wondering why Latex aligns my equations like this.

The code:

\documentclass[fleqn, 12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath, amsthm, amssymb, mathtools}
\usepackage[margin=1.25in, showframe]{geometry}
\setlength{\mathindent}{0pt}
\setlength{\jot}{1ex}
\usepackage[nodisplayskipstretch]{setspace}
\setstretch{1.25}

\begin{document}
\begin{align*}
I=O+P \\
\int^o_p (x-1)^2 \,dx
\end{align*}

\begin{aligned} I=O+P \\ \int^o_p (x-1)^2 \,dx \end{aligned}

\begin{align*}
u_1\in U_1 \Rightarrow u_1\in U_1 \cup U_2 \\
u_2\in U_2 \Rightarrow u_2\in U_1 \cup U_2 \\
\textrm{Since } U_1 \cup U_2 \textrm{ is a subspace of $V$, } u_1+u_2\in U_1 \cup U_2 \\
\Rightarrow u_1+u_2\in U_1 \textrm{ or } u_1+u_2\in U_2
\end{align*}

\begin{align*}
R_{eq}=\left(\frac{1}{R_1}+\frac{1}{R_2}\right)^{-1} &= \left(\frac{1}{3000}+\frac{1}{1000}\right)^{-1} \\
&= 750
\end{align*}

\end{document}


I suggest that you (a) replace the first two align* environments with gather* environments and (b) start using alignment points in the aligned environment (and place a \noindent directive before the aligned environment).

As an alternative to suggestion (a) above, consider (a') keep using the align* environments but start using alignment markers (&), one at the start of both lines in the first align* environment, and one at the start of all four lines in the second align* environment.

sometimes, I get weird indentations, at the beginning of lines depending on the environment and I sometimes have my text left aligned while right aligned (see third block of equation) and I was wondering why Latex aligns my equations like this.

That would be because you're not using a single alignment marker in that align* environment. See suggestion (a') above.

\documentclass[fleqn, 12pt]{article}
%% \usepackage[utf8]{inputenc} % <-- that's the default nowadays
\usepackage[margin=1.25in, showframe]{geometry}

\usepackage{mathtools, amsthm, amssymb}
\setlength{\mathindent}{0pt}
\setlength{\jot}{1ex}

\usepackage[nodisplayskipstretch]{setspace}
\setstretch{1.25}

\begin{document}

\begin{gather*}
I=O+P \\
\int^0_p (x-1)^2 \,dx
\end{gather*}

\noindent % <-- important
\begin{aligned} &I=O+P \\ &{\int^0_p (x-1)^2 \,dx} \end{aligned}

\begin{gather*}
u_1\in U_1 \Rightarrow u_1\in U_1 \cup U_2 \\
u_2\in U_2 \Rightarrow u_2\in U_1 \cup U_2 \\
\textrm{Since } U_1 \cup U_2 \textrm{ is a subspace of $V$, } u_1+u_2\in U_1 \cup U_2 \\
\Rightarrow u_1+u_2\in U_1 \textrm{ or } u_1+u_2\in U_2
\end{gather*}

\begin{align*}
R_{\mathrm{eq}}=\left(\frac{1}{R_1}+\frac{1}{R_2}\right)^{-1}
&= \left(\frac{1}{3000}+\frac{1}{1000}\right)^{-1} \\
&= 750
\end{align*}

\end{document}


Mico has suggested better markup, so here I concentrate on explaining the output that you get. Mainly it is (almost always) user error to use align with no alignment points.

\documentclass[fleqn, 12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath, amsthm, amssymb, mathtools}
\usepackage[margin=1.25in, showframe]{geometry}
\setlength{\mathindent}{0pt}
\setlength{\jot}{1ex}
\usepackage[nodisplayskipstretch]{setspace}
\setstretch{1.25}

\begin{document}

Math block left alighted (fleqn, and mathindent is 0), math within the block fush right (as no \&.)
\begin{align*}
I=O+P \\
\int^o_p (x-1)^2 \,dx
\end{align*}

Math block indented by paragraph indent as inline. Each line of math within the block flush right as no \&

\begin{aligned} I=O+P \\ \int^o_p (x-1)^2 \,dx \end{aligned}

As case 1, math block flush left but each line is flush right within the block.
\begin{align*}
u_1\in U_1 \Rightarrow u_1\in U_1 \cup U_2 \\
u_2\in U_2 \Rightarrow u_2\in U_1 \cup U_2 \\
\textrm{Since } U_1 \cup U_2 \textrm{ is a subspace of $V$, } u_1+u_2\in U_1 \cup U_2 \\
\Rightarrow u_1+u_2\in U_1 \textrm{ or } u_1+u_2\in U_2
\end{align*}

Math block flush left , aligned on =
\begin{align*}
R_{eq}=\left(\frac{1}{R_1}+\frac{1}{R_2}\right)^{-1} &= \left(\frac{1}{3000}+\frac{1}{1000}\right)^{-1} \\
&= 750
\end{align*}

\end{document}