Function = one input → one output
f(3), f(x)=3x+1 → 3(3)+1 = 10
Couple = (x, f(x))
Domain = inputs · Range = outputs
Codomain = the bigger set
f(x) = number → set equal, solve
f(x) = g(3) → do g(3) first
f(x) = g(x) → set rules equal
fg(x) = f(g(x)) — inner first
Order matters: fg ≠ gf
Algebraic: whole inner in brackets