Question 178
Prove that for all real numbers $x$ and $y$ the inequality
$$
\cos \left(x^{2}\right)+\cos \left(y^{2}\right)-\cos (x y)\lt 3
$$
holds.
$$
\cos \left(x^{2}\right)+\cos \left(y^{2}\right)-\cos (x y)\lt 3
$$
holds.
MathNet 2007-part-02 — 187 problems (EN–VI) Question 178
Math Olympiad Vault · MathNet 2007-part-02 — 187 problems (EN–VI)