y)=3 x^{2} y+y^{2}-3 x^{2}-3 y^{2}+2 \\ \text { (11) } z=f(x, y)=-x^{2}+2.5 y^{2}+10 x y-16 x-30 y \end{array} 2. Show that the function z=f(r,y) = 2x2 + y +3xy – 3y – 5x + 8 has a single critical(stationary) point at x = -1 and y = 3 and that is a saddle (10 points).

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