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In this lesson, you will learn what a function is and how the vertical line test is used to determine if an equation or graph represents a function or not. In a function, we take input values, or independent variables, called "x" values, and perform a calculation on them. The output value is called f(x). If we collect the input and output values as points on an x,y plane, functions have a one-to-one relationship. For every input value x, there is only one output value f(x). Because of this, a vertical line can only slice the graph of a function at one intersection point. If this isn't true, then it isn't a function.