[Solved] How do I increase the space below and above the equations in the cells of the long table

itc Asks: How do I increase the space below and above the equations in the cells of the long table
Below is my code:

Code:
documentclass[11pt,oneside,openany]{book}
usepackage[a4paper, left=1.5cm, right=1.5cm, top=3cm, bottom=3cm]{geometry}
usepackage{amsmath}
usepackage{amsfonts}
usepackage{amssymb}
usepackage{amsthm}
usepackage{longtable}
begin{document}
begin{longtable}{|p{1.7cm}|p{3.6cm}|p{2.8cm}|p{4cm}|}
    hline
        bfseries $ t $&bfseries $ D_{t} $&bfseries $ ABCD  $&bfseries $ sdf$ \ hline hline
    endfirsthead
    hline 
        bfseries $ t $&bfseries $ D_{t} $&bfseries $ TV=P_{n_{1}+n_{2}}  $&bfseries $ PV $ \ hline hline
    endhead
    hline multicolumn{4}{r}{emph{Continued on next page}}
    endfoot
    hline
    endlastfoot
    1& &  &$ dfrac{D_{1}}{(1+K_{r_{1}})} $ \  
    hline 
    $x $&& &$ dfrac{D_{1}}{(1+K_{r_{1}})^{n_{1}}} $ 
end{longtable}
end{document}


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