7 For each value of \(t\), the surface \(S _ { t }\) has equation \(z = t x ^ { 2 } + y ^ { 2 } + 3 x y - y\).
- Verify that there are no stationary points on \(S _ { t }\) when \(t = \frac { 9 } { 4 }\).
- Determine, as \(t\) varies, the nature of any stationary point(s) of \(S _ { t }\).
(You do not have to find the coordinates of the stationary points.)
\section*{OCR}
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