Question 129
Prove that for any real numbers $x$ and $y$ the inequality
$$
|\cos x| + |\cos y| + |\cos(x+y)| \ge 1
$$
holds.
$$
|\cos x| + |\cos y| + |\cos(x+y)| \ge 1
$$
holds.
MathNet 2006 — Part 04 Question 129
Math Olympiad Vault · MathNet 2006 — Part 04