Possible derivation:
d/dx(log(abs(cos(π/x))))
Using the chain rule, d/dx(log(abs(cos(π/x)))) = ( dlog(u))/( du) ( du)/( dx), where u = abs(cos(π/x)) and d/( du)(log(u)) = 1/u:
= (d/dx(abs(cos(π/x))))/abs(cos(π/x))
Using the chain rule, d/dx(abs(cos(π/x))) = ( dabs(u))/( du) ( du)/( dx), where u = cos(π/x) and d/( du)(abs(u)) = u/abs(u):
= ((cos(π/x) d/dx(cos(π/x)))/(abs(cos(π/x))))/abs(cos(π/x))
Simplify the expression:
= (cos(π/x) (d/dx(cos(π/x))))/abs(cos(π/x))^2
Using the chain rule, d/dx(cos(π/x)) = ( dcos(u))/( du) ( du)/( dx), where u = π/x and d/( du)(cos(u)) = -sin(u):
= -d/dx(π/x) sin(π/x) cos(π/x)/abs(cos(π/x))^2
Factor out constants:
= -(cos(π/x) sin(π/x))/abs(cos(π/x))^2 π d/dx(1/x)
Use the power rule, d/dx(x^n) = n x^(n - 1), where n = -1.
d/dx(1/x) = d/dx(x^(-1)) = -x^(-2):
= -(π cos(π/x) sin(π/x))/abs(cos(π/x))^2 (-1)/(x^2)
Simplify the expression:
= (π cos(π/x) sin(π/x))/(x^2 abs(cos(π/x))^2)
Simplifying powers, abs(cos(π/x))^2 = cos^2(π/x):
= (π tan(π/x))/x^2 [Courtesy of Mathematica 11 Home Edition]