\[x^{2} - 8x + c = 0 \Longrightarrow\]
\[\Longrightarrow единственный\ корень\ при\ \]
\[D = 0.\]
\[D = ( - {8)}^{2} - 4 \cdot 1 \cdot c =\]
\[= 64 - 4c = 0\]
\[64 - 4c = 0\]
\[4c = 64\]
\[c = 16\]
\[Ответ:при\ c = 16.\]