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The function $f (x) = \sin^{4} x + \cos^{4} x$ increases if

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Correct option is B

f(x)=sin4x+cos4x = cos2x+sin2x2−2sin2xcos2x =1−12sin22x = 1−141−cos4x=34+14cosx∴f1x=−sin4xfor f(x) to be increasing f1(x)>0 ⇒sin4x<0 ⇒ π<4x<2π ⇒π4

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The function fx=sin4x+cos4x increases if

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The function $f (x) = \sin^{4} x + \cos^{4} x$ increases if