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The core idea, one worked example, and short practice.
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A function is a rule. You put a number in, the rule does something to it, and one number comes out. Think of a vending machine: press the same button and you always get the same snack.
The name is read f of x. The letter is the name of the function, is the number you put in, and is the number that comes out. It does not mean times .
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To evaluate a function, wherever you see , write the input number instead, then do the arithmetic. Multiply before you add.
Many SAT functions look like . The number multiplied by is the slope, or rate of change. The number standing alone is the y-intercept, or starting value.