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Given y= cos^4 (2x)
We need to find dy/dx
We will use the chain rule to find the derivative.
Let u= cos2x ==> u' = -2sin(2x)
==> y= u^4
==> y' = 4u^3 * u'
Now we will substitute with u= cos2x
==> y' = 4(cos^3 2x) * -2sin2x
==> dy/dx = -8sin2x*cos^2 2x
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