Problem $1.$ For $1$ to $3$ below, find the stationary points by setting the first partial derivatives

equal to zero and solving the resulting equations. Determine whether the points obtained are

maxima or minima.

$f(x)=3x_1^2+4x_2^2-x_1{x}_2-5x_1-8x_2$

$f(x)=-2x_1^2-x_2^2-4x_3^2-3x_1{x}_3+3x_2{x}_3-x_1-2x_2-3x_3$

$f(x)=-2x_1^2-x_2^2-4x_3^2-8x_1{x}_3+4x_2{x}_3-x_1-2x_2-3x_3$