Which trigonometric function requires a domain restriction of [-pi/2,pi/2]?

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}

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