\usepackage{fitch} \usepackage{amsmath} \begin{document} $\Large\begin{nd} \hypo {h1} {\lnot(P\land Q)} \open \hypo {h2} {\lnot(\lnot P\lor\lnot Q)} \open \hypo {h3} {P} \open \hypo {h4} {Q} \have {pq} {P\land Q} \ai{h3,h4} \have {h4-1} {\bot} \ie{h1,pq} \close \have {h3-1} {\lnot Q} \ii{h4-{h4-1}} \have {h3-2} {\lnot P\lor\lnot Q} \oi{{h3-1}} \have {h3-3} {\bot} \ie{h2,{h3-2}} \close \have {h2-1} {\lnot P} \ii{h3-{h3-3}} \have {h2-2} {\lnot P\lor\lnot Q} \oi{{h2-1}} \have {h2-3} {\bot} \ie{h2,{h2-2}} \close \have {h1-1} {\lnot\lnot(\lnot P\lor\lnot Q)} \ii{h2-{h2-3}} \end{nd}$ \end{document}