calculus
posted by NYSNC .
Verify the identity:
arctanx + arctan(1/x)=∏/2
using calculus theory.
(Hint: Differentiate the left hand side of the identity)

Actually, since tan(pi/2x) = cot(x)
and cot = 1/tan it falls right out.
Or, using the sum of tangents formula,
tan(arctanx + arctan(1/x))
= [tan(arctan(x)) + tan(arctan(1/x))][1  tan(arctan(x))*tan(arctan(1/x))]
= [x + 1/x]/[1  x*1/x] = (x + 1/x)/0 = oo
tan pi/2 = oo