Question 13
Let $0 \leqslant a, b, c, d, e \leqslant 1$ be real numbers. Show that
$$
(1+a+b+c+d+e)^2 \geqslant 4\left(a^2+b^2+c^2+d^2+e^2\right)
$$
$$
(1+a+b+c+d+e)^2 \geqslant 4\left(a^2+b^2+c^2+d^2+e^2\right)
$$
MathNet 2015-part-03 — 191 problems (EN–VI) Question 13
Math Olympiad Vault · MathNet 2015-part-03 — 191 problems (EN–VI)