18 À :: cos 4 x cos 2 x = 0                                        Á :: [π/2;2π] 

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À :: `+`(`*`(`/`(1, 3), `*`(Pi, `*`(k)))), `in`(k, Z) Á :: `*`(`+`(`*`(2, `*`(Pi))), `/`(1, 3)); 1; Pi; 1; `*`(`+`(`*`(4, `*`(Pi))), `/`(1, 3)); 1; `*`(`+`(`*`(5, `*`(Pi))), `/`(1, 3)); 1; `+`(`*`(2, `*`(Pi)))