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vertical line test
In mathematics, the vertical line test is a visual way to determine if a curve is a graph of a function or not. A function can only have one output, ''y'', for each unique input, ''x''. If a vertical line intersects a curve on an ''xy''-plane more than once then for one value of ''x'' the curve has more than one value of ''y'', and so, the curve does not represent a function. If all vertical lines intersect a curve at most once then the curve represents a function.〔
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To use the vertical line test, take a ruler or other straight edge and draw a line parallel to the ''y''-axis for any chosen value of ''x''. If the vertical line you drew intersects the graph more than once for any value of ''x'' then the graph is not the graph of a function. If, alternatively, a vertical line intersects the graph no more than once, no matter where the vertical line is placed, then the graph is the graph of a function. For example, a curve which is any straight line other than a vertical line will be the graph of a function. As another example, a sideways parabola (one whose directrix is a vertical line) is not the graph of a function because some vertical lines will intersect the parabola twice.
==See also==
*Horizontal line test
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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