solve for the second derivative and set to zero
f"(x) = ((x^2-4)*sin(2x)- [(2x)(1+cos^2(x))]/ ((1+cos^2(x))^2)
i don't know how to set it to zero and solve i get this:
(x^2-4)(sin(2x))= (2x)(1+cos^2(x))
thanks for help
I reduced your expression to
(x^2-4)*sin(2x)- (2x)/(1+cos^2(x))=0
(x^2-4)*sin(2x) = (2x)/(1+cos^2(x))
(x^2-4)*sin(2x)(1+cos^2(x)) = 2x
which is not what we are to get.
I noticed that the very first and very last brackets are not really needed.
Perhaps some of those brackets are misplaced.