Question 43
Let $a$, $b$, $c$, $d$ be positive real numbers such that
$$
a^2 + b^2 + c^2 + d^2 = 4.
$$
Prove that there are two of $a$, $b$, $c$, $d$ with sum greater or equal to $2$.
$$
a^2 + b^2 + c^2 + d^2 = 4.
$$
Prove that there are two of $a$, $b$, $c$, $d$ with sum greater or equal to $2$.