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Functions assign a single unique output for each of their inputs. In this video, Sal gives many examples of various kinds of functions.
Jesse Roe and Sal talk about the difference between equations and functions
Relations and Functions
Linear Function Graphs
Sal evaluates f(x)=49-x^2 at x=5.
Sal evaluates a function at x=-1 using the graph of that function.
Evaluate functions at a specific input value, given either the functions' formula or their graph.