方程式排列不正确

Question

问题：

我怎样才能保持下一页方程的连续性？第二个问题，我的一个方程太长了，我怎样才能打破它？

梅威瑟：

\documentclass[12pt,fleqn]{beamer}
\usetheme{AnnArbor}
\usepackage{setspace}
\usepackage{amsmath}
\usepackage[utf8]{inputenc}
%\setstretch{1.0}
\usecolortheme{beaver}
\usefonttheme{professionalfonts} % using non standard fonts for beamer
\usefonttheme{serif} % default family is serif
\addtobeamertemplate{frametitle}{}{\vspace{-0.4em}} % decrease
\makeatletter
\newcommand{\Pause}[1][]{\unless\ifmeasuring@\relax
    \pause[#1]%
    \fi}
\makeatother

\title[pqr lmn] %optional
{Chapter}

\subtitle{Lecture - 0}

\author[X. Y. Z] % (optional, for multiple authors)
{x.~y.~z \\ abc \\ def}
%\date[\today] % (optional)

\begin{document}
    
\frame{\titlepage}
    \begin{frame}
        %\setstretch{1.0}
        \textbf{Find nth order derivative of} $\boldsymbol{\cos x \cos 2x \cos 3x}$
        \begin{align*}
        y&=\cos x \cos 2x \cos 3x\\[8pt]
        &=\dfrac{1}{2}\cos x \left(2\,\cos 3x \cos 2x\right)\\[12pt]
        &=\dfrac{1}{2}\,\cos x \,\left(\cos 5x + \cos x\right)\\[12pt]
        &=\dfrac{1}{4}\,\left[2 \cos 5x \cos x + 2\cos^2 x\right]\\[12pt]
        &=\dfrac{1}{4}\,\left[\cos 6x + \cos 4x + 1 + \cos 2x\right]
        \end{align*}
    \end{frame}
    
\begin{frame}
        %\setstretch{1.0}   
        $$\therefore\;\;y_n=\dfrac{1}{4}\,\left[\left\{\cos 6x\right\}_n + \left\{\cos 4x\right\}_n + \left\{1\right\}_n + \left\{\cos 2x\right\}_n\right]$$
        $$=\dfrac{1}{4}\,\left[6^n\,\left\{\cos 6x+\dfrac{n\pi}{2}\right\} + 4^n\,\left\{\cos 4x+\dfrac{n\pi}{2}\right\} + 0 + 2^n\,\left\{\cos 2x+\dfrac{n\pi}{2}\right\}\right]$$
\end{frame}
\end{document}