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Maka,
f'(x) = 0 - 2.cos(2x)
Maka,
-2.cos(2x) > 0
cos(2x) < 0 [Tanda dibalik]
cos(2x) < cos(90)
Artinya nilai cosinus negatif pada:
90 < 2x < [360]-90
90 < 2x < 270
45 < x < 135