证明恒等式arctanx+arccotx=π/2 , f(x) = arctanx+arccotx, 则有f'(x) =

证明恒等式arctanx+arccotx=π/2 , f(x) = arctanx+arccotx, 则有f'(x) = 1/(1 + x^2) - 1/(1 + x^2) = 0,
f(x) = arctanx+arccotx,
则有f'(x) = 1/(1 + x^2) - 1/(1 + x^2) = 0,
所以由那个定理,f(x)是常数.把x = 1代入,得到
f(1) = arctan 1 + arccot 1 = π/2
所以f(x) = arctanx + arccotx = π/2
这个题的答案为什么要令f(1)=π/2?可以随便假设一个常数?