Problem #PRU-116563

Problems Algebra Trigonometry Identical transformations (trigonometry) Methods Proof by contradiction Calculus Real numbers Rational and irrational numbers

Problem

Does there exist a real number \({\alpha}\) such that the number \(\cos {\alpha}\) is irrational, and all the numbers \(\cos 2{\alpha}\), \(\cos 3{\alpha}\), \(\cos 4{\alpha}\), \(\cos 5{\alpha}\) are rational?