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Vertical Line Test

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About This MicroSim

Move your mouse across the graph to drag the vertical line. The red dots show where the line intersects the curve. If the line ever hits the curve more than once, it's not a function!

Lesson Plan

Learning Objective: Students will use the vertical line test to determine whether a given graph represents a function.

Key Insight: A function can only have ONE output for each input. If a vertical line (representing a single x-value) crosses the graph twice, that x has two outputs - which breaks the function rule!