\mdfsetup{splitbottomskip=0cm,splittopskip=0cm,
innerrightmargin=0cm,innertopmargin=0cm,%
innerlinewidth=0pt,outerlinewidth=0pt,
middlelinewidth=0pt,backgroundcolor=lila,
tikzsetting={draw=yellow,line width=0pt,%
dashed,%
dash pattern= on 10pt off 5pt},
rightline=false,bottomline=false}

\begin{mdframed}
\df \textbf{(Transformacja Boxa-Coxa)}
$$f_{\lambda}(X_t)=\left\{
\begin{array}{cccc}
\frac{1}{\lambda}(X_t^{\lambda} - 1)&\mbox{gdy}&\lambda > 0,&X_t\geq 0 \\
\ln X_t&\mbox{gdy}&\lambda=0,&X_t >0
\end{array}
\right.$$
\end{mdframed}