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Answer :

Answer:

40° or 2π/9 rad

Step-by-step explanation:

Let f(x) = 0 and solve

2cos (3x) + 1 = 0

2cos (3x) = -1

cos (3x) = -1/2

3x = arccos (-1/2) = 120

x = 40° or 2π/9 rad

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