The trigonometric function which requires a domain restriction of {-π/2,π/2}
is tan x , sec x
tan x = (sin x)/(cos x) and sec x = 1/(cos x)
cos x becomes zero at {-π/2,π/2}
see the attached figure
tan x ⇒⇒⇒ blue graph
sec x ⇒⇒⇒ red graph
as shown both of the function undefined at {-π/2,π/2}
