Consider the set $S$ of points $(x, y) \in R^2$ which minimize the real valued function

$$ f(x, y)=(x+y-1)^2+(x+y)^2 $$

Which of the following statements is true about the set $S$ ?

Let $f(x)$ be a real-valued function such that $f^{\prime}\left(x_0\right)=0$ for some $x_0 \in(0,1)$ and $f^{\prime \prime}\left(x_0\right)>0$ for all $x \in(0,1)$. Then $f(x)$ has

Suppose the circles $x^2+y^2=1$ and $(x-1)^2+(y-1)^2=r^2$ intersect each other orthogonally at the point $(u, v)$. Then $u+v=$ $\_\_\_\_$ .