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Questions  

If k=sin6x+cos6x, then k belongs to the interval

a
[7/8,5/4]
b
[1/2,5/8]
c
[1/4,1]
d
none of these

detailed solution

Correct option is C

We have,k=sin6⁡x+cos6⁡x⇒ k=sin2⁡x+cos2⁡xsin4⁡x+cos4⁡x−sin2⁡xcos2⁡x⇒ k=1−3sin2⁡xcos2⁡x⇒ k=1−34sin2⁡2xNow,0≤34sin2⁡2x≤34, for all x⇒ −34≤−34sin2⁡2x≤0, for all x⇒ 1−34≤1−34sin2⁡2x≤1, for all x⇒ 14≤1−34sin2⁡2x≤1, for all x⇒ 14≤k≤1

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