Rewrite the expression as an algebraic expression in x.

cos (tan-1 x)

It's hard to type and hard to read the "inverse tangent" function, as you've seen (above).

So, use "arctan x" instead.

Then the problem becomes: "differentiate cos (arctan x)."

You must apply first the rule for differentiating the cosine function, and next apply the rule for differentiating the arctan function:

(d/dx) cos (arctan x) = - sin (arctan x) * [1 / (1+x^2) ]