使用标签对齐方程式

Question

\documentclass{article}
\usepackage[a4paper, total={6in, 8in}]{geometry}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{fourier}
\usepackage{mathtools}
\usepackage{amsmath}
\usepackage[english]{babel}
\usepackage{graphicx}
\usepackage{marginnote} 
\begin{document}
$$ KP = e^{-\lambda} \Big( \frac {\nu'}{r} +\frac {1}{r}\Big) -\frac {1}{r^2}$$
    \begin {gather}
    \Rightarrow \frac {dP}{dr} = - \frac { P+\rho (P) c^2}{r(r-2u)} \Big[ \frac {K}{2} P r^3 +u \Big] \tag {**}
    \end {gather}
\end {document}

我怎样才能对齐它们？

Answer 1

\documentclass{article}
\usepackage{mathtools}
\begin{document}
\begin{align*}
  KP &= e^{-\lambda} \Big( \frac {\nu'}{r} +\frac {1}{r}\Big) -\frac {1}{r^2} \\[2ex]
 \Rightarrow \frac {dP}{dr} &= - \frac { P+\rho (P) c^2}{r(r-2u)} \Big[ \frac {K}{2} P r^3 +u \Big] \tag {**}
\end {align*}
\end{document}