Understanding how to write a quadratic equation from a graph is an essential skill in algebra, and it can be accomplished using various forms of the equation. In this article, we will discuss two primary methods for finding the equation of a quadratic function from a graph: using the vertex form and the standard form of the equation. These methods are straightforward once you understand the concepts of the vertex, coefficients, and graph points.

how to write a quadratic equation from a graph
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Using Vertex Form

The vertex form of a quadratic equation is given by y = a(x – h)2 + k, where (h, k) is the vertex of the parabola. If the graph provides the vertex point, it becomes significantly easier to write the quadratic equation using this form. Here’s how:

  • Identify the vertex point (h, k) from the graph.
  • Substitute h and k into the vertex form of the equation.
  • Use another point (x, y) on the graph to substitute these values into the equation and solve for the coefficient ‘a’.

For example, if the vertex of the graph is (2, 3) and another point on the graph is (4, 7), you would substitute these values into the vertex form, y = a(x – 2)2 + 3, and solve for ‘a’ to find the specific quadratic equation.

how to write a quadratic equation from a graph
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Using Standard Form

When the vertex is not known, and instead you have three points on the graph, you would use the standard form of the quadratic equation, y = ax2 + bx + c. This method involves the following steps:

  • Set up a system of equations using the coordinates of the three points.
  • Solve for the coefficients a, b, and c using substitution or elimination methods.
  • Once the values of a, b, and c have been determined, substitute them back into the standard form.

For instance, if the points (1, 2), (3, 10), and (5, 18) are given, you would set up three equations with these points, solve for the coefficients, and substitute these values back into the standard form to get the quadratic equation.

how to write a quadratic equation from a graph
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Graphing Quadratic Functions

Knowing how to write a quadratic equation from a graph also involves understanding the overall structure of quadratic graphs. Regardless of whether you use the vertex form or standard form, the process of graphing involves:

  • Finding the vertex, which can serve as a starting point.
  • Determining the axis of symmetry, which is the vertical line that passes through the vertex.
  • Plotting additional points on either side of the axis to form the shape of the parabola.

In practice, the vertex form is often more intuitive when the vertex is given, while the standard form is useful when multiple points are known without the vertex.

Practice Makes Perfect

To become proficient in writing quadratic equations from graphs, regular practice with different sets of points and graphs is crucial. Methods such as using the vertex form for the vertex and an additional point, or the standard form for three points, require familiarity and repeated application.

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