# [Tex/LaTex] the difference between aligned in displayed mode and starred align

alignamsmath

I am confused in choosing one of the following approaches.

\documentclass{article}
\usepackage{siunitx}
%\sisetup{detect-all}
\usepackage{amsmath}
\parindent=0pt\relax
\begin{document}
The following calculation is a trivial example
\begin{aligned} q &=CV \\ &=\num{3e-20x1000} \\ &=\num{3e-17} \end{aligned}
The following calculation is a trivial example
\begin{align*}
q       &=CV                                \\
&=\num{3e-20x1000}  \\
&=\num{3e-17}
\end{align*}

\end{document}


What is the difference between aligned in displayed mode and starred align?

Edit:

If there is a difference and we have to show the substitution steps above or algebraic operation steps below, which one we have to use based on the internationally adopted convention?

\documentclass{article}
\usepackage{amsmath}

\begin{document}
The following calculation is a trivial example
\begin{aligned} a(x+b)^2 - ab^2 &= a(x^2 + 2bx + b^2) - ab^2\\ &= ax^2 + 2abx + ab^2 - ab^2\\ &= ax^2 + 2abx \end{aligned}

The following calculation is a trivial example
\begin{align*}
a(x+b)^2 - ab^2 &= a(x^2 + 2bx + b^2) - ab^2\\
&= ax^2 + 2abx + ab^2 - ab^2\\
&= ax^2 + 2abx
\end{align*}

\end{document}


Your first version is essentially a single equation (that contains several lines of aligned equations, but that is secondary). Your second version consists of several equations. This alters the spacing above and below that:

\documentclass{article}
\usepackage{amsmath}
\parindent=0pt\relax
\begin{document}
The following calculation is a trivial example
\hrule
\begin{aligned} q &=CV \\ &=3e-20x1000 \\ &=3e-17 \end{aligned}
\hrule

\bigskip

\hrule
\begin{align*}
q       &=CV                                \\
&=3e-20x1000  \\
&=3e-17
\end{align*}
\hrule
\end{document}


yields:

Usually you want the larger spacing around a block of several equations. Your second version is also conceptually clearer.