You can put this solution on YOUR website! The domain is the set of inputs to a funtion.
f(x)=y where y is dependent on x. x is the input, y is the output.
Think of a function like a box that has a slot on one end, a hopper on the other end, and a hand crank in the middle. You put values into the slot (the values) and turn the crank. The function box then applies the rule that defines that function, and a function value (the values) comes out in the hopper.
The domain of a function is the set of values that you are allowed to put into the slot. Some functions allow you to put any value into them and get a valid answer out the other end, while other functions restrict you from putting in certain values.
For example, you would restrict the domain for any value that would cause a denominator in the function to equal zero; as in the situation where you had in a denominator in the function definition, you would not allow the value 3 to be included in the domain. There are other situations where you must restrict the domain in other ways -- it just depends on the particular function and over what set of numbers you want the function defined.
While we're at it, let's also discuss the range of a function. The range is the set of numbers that could possibly come out of the output hopper for all possible elements contained in the domain.
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