\[\boxed{\text{311\ (311).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[Квадратное\ уравнение\ не\ \]
\[имеет\ корней,\ когда\ D < 0.\]
\[\textbf{а)}\ 2x^{2} + tx + 18 = 0\]
\[D = t^{2} - 4 \cdot 2 \cdot 8 < 0\]
\[t^{2} - 144 < 0\]
\[(t - 12)(t + 12) < 0\]
\[t \in ( - 12;12).\]
\[\textbf{б)}\ 4x^{2} + 4tx + 9 = 0\]
\[D = 4t^{2} - 4 \cdot 9 = 4t^{2} - 36\]
\[4t^{2} - 36 < 0\ \ \ \ \ |\ :4\]
\[t^{2} - 9 < 0\]
\[(t - 3)(t + 3) < 0\]
\[t \in ( - 3;3).\]