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How to find the Vertex of a Parabola

Updated on March 27, 2011

A parabola is a curve which is given by a quadratic equation: y = ax2 + bx + c

Every parabola contains a vertex.  A vertex is the maximum or minimum point on the curve, at which the gradient of the curve is zero. This article is a tutorial in how to find the coordinates of this vertex


How to Find the Vertex Coordinates

1. Write the parabola equation in the form: y = ax2 + bx + c (where a, b and c are constants)

2. Find the gradient of the parabola by differentiating the parabola equation:

y = ax2 + bx + c differentiates to give dy/dx = 2ax + b

3. Find the value of x which gives a gradient of zero:

dy/dx = 2ax +b = 0 when x = -b / 2a

4. Find the value of y which corresponds to this x, by putting x = -b / 2a back into the parabola equation:

y = a (-b / 2a)2 + b (-b / 2a) + c

= b2 / 4a - b2 / 2a + c

= -b2 / 4a +c

5.  The co-ordinates of the vertex are therefore (-b / 2a, -b2 / 4a + c)

An example of a parabola: y=-2x^2-4x+6
An example of a parabola: y=-2x^2-4x+6

Example

An example parabola is shown in the diagram.

1. The equation of this parabola is y = -2x2 - 4x + 6

2. The gradient is dy/dx = -4x - 4

3. The gradient is zero when x = -1

4. When x = -1, y = -2(-1)2 - 4(-1) + 6 = -2 + 4 + 6 = 8

5. The vertex coordinates are therefore (-1, 8)

Mathematics Textbooks

Mathematical Methods for Physics and Engineering: A Comprehensive Guide
Mathematical Methods for Physics and Engineering: A Comprehensive Guide

This is the book I used throughout my degree. It provides a great grounding in college-level mathematics for anyone studying math or a physical science.

 

Maximum or Minimum?

One last question remains to be answered. How do we know whether the vertex we have found from the equation is a maximum or a minimum point?

There is a simple test, which can be performed as follows:

1. Take the equation for the gradient, dy/dx, and differentiate it again by x, to get d2y/dx2.

2.  This quantity d2y/dx2 is the rate of change of the gradient.

If d2y/dx2 <0, then the gradient is decreasing as x increases. This means that the gradient of the parabola is positive on the left of the vertex and negative on the right. The vertex is a maximum.

If d2y/dx2 >0, then the gradient is increasing as x increases. This means that the gradient of the parabola is negative on the left of the vertex and positive on the right. The vertex is a minimum.

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    • La Papillon profile image

      La Papillon 6 years ago from Melbourne, Australia

      Interesting. This takes me back to my old school days, when I actually understood and enjoyed math/calculus.

      Cheers, Louise :)

    • Simone Smith profile image

      Simone Haruko Smith 6 years ago from San Francisco

      If only I had found such a helpful guide when I was still in high school! GREAT Hub!

    • Spirit Whisperer profile image

      Xavier Nathan 6 years ago from Isle of Man

      Another very clearly explained topic. Might I be so bold as to suggest you might add some applications of this Mathematics. Voted up!

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