Question 129
Solve
$$
\cos(2x) + \cos(2y) + 2 \sin x \sin y + 2 \cos x \cos y = 4.
$$
(Petar Bakić)
$$
\cos(2x) + \cos(2y) + 2 \sin x \sin y + 2 \cos x \cos y = 4.
$$
(Petar Bakić)
MathNet 2019-part-02 — 184 problems (EN–VI) Question 129
Math Olympiad Vault · MathNet 2019-part-02 — 184 problems (EN–VI)