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cos4x<-(√2/2)

cos(4x)= -sqrt(2)/2

4x = 3π/4 + 2πk

4x = - 3π/4 + 2πk = 5π/3 + 2πk

x = 3π/16 + πk/2

x = 5π/16 + πk/2

cos(4x) < -sqrt(2)/2

x € (3π/16 + πk/2 ; 5π/16 + πk/2) ; k € Z