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Well, since cos π/2 = 0, and 6 • π/12 = π/2, π/12 is one solution.

But, cos 3π/2 also = 0 and so does cos 5π/2 so dividing those by 6 gives two more answers in your given interval for Θ, π/4 and 5π/12

x^4 - 20x^2 + 19 = 0 replace y = x^2: y^2 - 20y + 19 = 0 factor and make certain for y: (y-a million)(y-19) = 0 y = a million or y = 19 replace x^2 = y: x^2 =a million or x^2 = 19 for that reason, the recommendations are: x = +OR- a million and x = +OR- sqrt(19)