use the identity (a+b)(a-b) = a^2 - b^2 with {(sec x)-1}{ (sec x +1) } to get
sec^2(x) - 1
we can now write expression as
{sec^2(x) -1} * cot (x)
and use the trig identity sec^2(x) - 1 = tan^2(x) to get expression as
tan^2(x)*cot(x) = tan^2(x) * {1/tan(x)} = tan(x)
