[Tex/LaTex] How to reduce space between two math environments

Question

I force to use two math environments in series, this will present a big space that I don't like, I saw this ansewer, he uses \\[-1ex] I don't want to use this method for each equation (I have a lot of equations), so is there any way to reduce the space between the two environments for the whole document?

Here is a simple example

\documentclass[a4paper,14pt]{extreport}
\usepackage{amsmath}
\begin{document}
\begin{align*}
     r &= \sqrt{x^2 + y^2} \\
     f &= m a \\
     A = \frac{\pi}{4} d^2
\end{align*}  
\begin{align*}
     \frac{\partial P}{\partial \theta} = \frac{P_{(i+1,j)} - P_{(i-1,j)}}{2 \Delta\theta}
\end{align*}
\end{document}

Answer 1

Consecutive math displays should be avoided. If you really must group aligned display math and non-aligned content, use a single display math environment (here, gather*) and use an internal math environment to perform the alignment (here, aligned):

\documentclass[a4paper,14pt]{extreport}
\usepackage{amsmath}
\begin{document}
\begin{gather*}
\begin{aligned}
     r &= \sqrt{x^2 + y^2} \\
     f &= m a \\
     A &= \frac{\pi}{4} d^2
\end{aligned} \\
     \frac{\partial P}{\partial \theta} = \frac{P_{(i+1,j)} - P_{(i-1,j)}}{2 \Delta\theta}
\end{gather*}
\end{document}