is an empty set.
contains exactly one element.
contains exactly two elements.
contains more than two elements.
f(x) + 2f(1/x) = 3x
Putting x as 1/x then f(1/x) + 2f(x) = 3(1/x)
Solving these two equations,
f(x) = x - 1/x
Now for f(x) = f(-x) ⇒ x - 1/x = -x + 1/x
⇒ 2x = 2/x ⇒ x² = 1 ⇒ x = ±1