Online Test Banks
Score higher
See Online Test Banks
eLearning
Learning anything is easy
Browse Online Courses
Mobile Apps
Learning on the go
Explore Mobile Apps
Dummies Store
Shop for books and more
Start Shopping

ACT Trick for Quadratics: How to Quickly Find the Vertex Location of a Parabola

To save time when graphing a quadratic function on the ACT Math test, you can quickly find the location of the vertex of the parabola in relation to the y-axis. Just use the following simple trick based on the variables a and b (in the terms ax2 and bx):

  • When the signs of a and b are the same (either both positive or both negative), the graph shifts to the left. That is, the vertex of the parabola is to the left of the y-axis.

  • When the signs of a and b are different (one is positive and the other is negative), the graph shifts to the right. In this case, the vertex of the parabola is to the right of the y-axis.

A good mnemonic for remembering this rule is that the word same and the word left both have four letters.

Example

A quadratic function y = ax2 + bx + c crosses the x axis at x = 4 and x = –2. Which of the following must be true?

image0.png

At first glance, you may think this difficult question isn’t even answerable. A quick sketch shows you that a lot of different parabolas could fit this description:

image1.jpg

The only thing you know for sure is that the parabola’s vertex is horizontally in the middle of these two points, so it’s somewhere on the line x = 1. Thus, the parabola (whatever it looks like) is shifted to the right, so a and b have different signs. So a does not equal b, making the correct answer Choice (C).

  • Add a Comment
  • Print
  • Share
blog comments powered by Disqus
Advertisement

Inside Dummies.com

Dummies.com Sweepstakes

Win $500. Easy.