How to Identify the Function from the Graph

In mathematics, a function is a rule or a relation that maps elements from one set, called the domain, to elements of another set, called the range. The primary characteristic that distinguishes a function from a general relation is that each element in the domain must be associated with exactly one element in the range.

How to Identify the Function from the Graph

Step-by-step Guide to Identify the Function from the Graph

Here is a step-by-step guide to identify the function from the graph:

Step 1: Foundational Grounding

  • Familiarize yourself with the basic definition of a function. Recall that a function assigns to every input exactly one output.

Step 2: Utilize the Vertical Line Test

  • The vertical line test is a fundamental tool for identifying functions. Draw or imagine vertical lines across the graph:
    • If any vertical line intersects the graph at more than one point, the graph doesn’t represent a function.
    • If every vertical line intersects the graph at most once, it is a function.

Step 3: Spot Special Cases

  • Some graphs, like those of vertical lines (e.g., \(x=3\)), will fail the vertical line test immediately. These are never functions.

Step 4: Analyze Continuous vs. Discontinuous Graphs

  • Continuous graphs have no breaks, jumps, or holes. If such a graph passes the vertical line test, it represents a continuous function.
  • Discontinuous graphs may have gaps or jumps. Each separate piece of the graph should be individually subjected to the vertical line test.

Step 5: Evaluate for Implicit Functions

  • Some functions, especially those involving circles or ellipses, might not be explicitly solved for one variable. Even if they appear complex, the vertical line test remains a valid tool.

Step 6: Understand Graph Behavior at Asymptotes

  • Vertical asymptotes can be tricky. A graph might approach but never cross a vertical asymptote, which can still be consistent with a function. However, if it crosses an asymptote more than once, it fails the function test.

Step 7: Investigate End Behavior and Horizontal Asymptotes

  • While the horizontal asymptotes and end behavior don’t directly determine if a graph is a function, they can give insights into the function’s type and characteristics.

Step 8: Distinguish One-to-One Functions with the Horizontal Line Test

  • While the vertical line test determines if a graph is a function, the horizontal line test determines if it’s a one-to-one function. If no horizontal line intersects the graph more than once, the function is one-to-one.

Step 9: Employ Graphing Tools and Software

  • Utilize tools like Desmos, GeoGebra, or graphing calculators to visualize and manipulate graphs. These can help in understanding complex graphs and verifying if they represent functions.

Step 10: Engage in Comparative Analysis

  • Compare the given graph with standard function shapes you know (e.g., linear, quadratic, sine, exponential). Recognizing similarities or patterns can aid in identification.

Step 11: Iterative Verification

  • Continuously re-evaluate as you analyze. Sometimes, initial perspectives might not capture all the nuances of a complex graph.

Step 12: Seek Peer Reviews

  • Sometimes, discussing with peers or presenting your analysis to others can help spot oversights or provide alternative viewpoints.

By systematically following this multifaceted approach, one can accurately identify whether a given graph, even one with high variation and complexity, represents a function. This process not only focuses on identification but also deepens one’s understanding of the nature and behavior of functions in general.


Given the following graph, determine whether the graph is a function or not.


Draw a vertical line across the graph such as the line drawn in the graph below. It intersects the graph at most once, So, it is a function.

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