Question

What is the y-value for f(x) = tan(90°)?

Answer 1

in a function we can say f(x) = y
y = f(x) = tan(90°)
y = f(x) = ∞ (undefined)

Answer 2

f(x) = tan90° ⇒ y = tan90°

tan90° non exist, therefore the y-value not exist.