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}
```

## Best Answer

Your first version is essentially a

singleequation (that contains several lines of aligned equations, but that is secondary). Your second version consists ofseveralequations. This alters the spacing above and below that:yields:

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