NDA Math Differentiation

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Question : 8 of 64
 
Marks: +1, -0
If y=cos‌x⋅cos‌4‌x⋅cos‌8‌x, then what is ‌
1
y
‌
dy
dx
at x=‌
Ï€
4
equal to?
[NDA-II 2021]
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