SOLUTION: If f(x)=cos nx, find f^(4)[pi/n]
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Question 460227: If f(x)=cos nx, find f^(4)[pi/n]
Answer by richard1234(7193) (Show Source): You can put this solution on YOUR website!
Find the fourth derivative by finding all the other derivatives using the chain rule (there is a pattern).
 = -n \sin(nx))
 = -n^2 \cos(nx))
 = n^3 \sin (nx))
} (x) = n^4 \cos (nx))
At x = pi/n,
} (\frac{\pi}{n}) = n^4 \cos (\pi) = -n^4)
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