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Easy homework help for functions in algebra or precalculus
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Do you need to determine whether a relation is a function? The process may sound difficult, but it's actually pretty easy. To know if a relation is a function, just examine the inputs and outputs. When you’re given a set of ordered pairs, check whether any inputs have multiple outputs. If so, the relation is not a function. You can also do the vertical line test to check whether a relation is a function. This wikiHow guide shows you how to know when a relation is a function.
Things You Should Know
- Relations are a set of inputs and outputs, often shown as ordered pairs or a table.
- Functions are relations in which each input has exactly one output.
- Check whether each input has one output to know if a relation is a function.
Steps
Section 1 of 3:
Definitions
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Functions occur when each input has exactly one output. Different inputs can have the same output value. An input can not be assigned to multiple output values.- If you’re looking for quadratic function information, check out our guides on finding the inverse and finding the max or min .
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Section 2 of 3:
Evaluate the Relation
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Put your ordered pairs into a table. If you were given a set of ordered pairs, putting them into a table is helpful for checking if the relation is a function. Skip this step if you already have a table of inputs and outputs.- Take the left value (the x value) of each ordered pair and place them vertically in the left column (input) of a 2 column table.
- Repeat for the right values (the y values), placing them in the right column (output).
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Check whether any inputs have multiple outputs. If an input has multiple outputs, the relation is not a function. -
Take a look at this non-function example. You’re given the following set of ordered pairs. Because the input 1 has two outputs, 2 and 7, this relation is not a function.- (1, 2)
- (2, 4)
- (3, 10)
- (1, 7)
- (5, 0)
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Check out this function example. You’re given the following set of ordered pairs. Because each input only has one output, this relation is a function. Functions can have different inputs assigned to the same output. In this case, the inputs 3 and 4 have both been assigned to 10.- (1, 2)
- (2, 4)
- (3, 10)
- (4, 10)
- (5, 0)
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Section 3 of 3:
Vertical Line Test
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Plot the ordered pairs on a graph. Place a dot at each given ordered pair coordinate on an XY plane. -
Move a vertical line across the graph. Use a straight-edge (like a ruler or another sheet of paper) to move a vertical line (parallel to the Y axis) across the entire graph. If the vertical line hits two or more points at the same X location, the relation is not a function.- In other words, if two or more points have the same X value and different Y values, the relation is not a function.
- The relation is a function if you move the vertical line across the graph without hitting two or more points at the same time.
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