How to Make a Line of Symmetry

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Making a line of symmetry is a lot more complicated than just drawing a straight line down a page. Make a line of symmetry with help from an experienced educator in this free video clip.

Part of the Video Series: Lessons in Math
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Video Transcript

Hi, my name is Marija. I'm a mathematician, and today I'm going to tell you how to make a line of symmetry. So we're going to talk about parabolic functions today and let's start with the equation X squared plus 2X plus 3. So when you are going to look for the line of symmetry or the axis of symmetry that's going to split this parabola into two equal halves, we're going to be using the equation X equals negative B over 2A and now we're going to identify what A, B and C are. So A is the coefficient of X squared which right now is imaginary so it's a 1. B is the coefficient of X and C is the constant that's at the end of your quadratic. So all we're going to do is substitute into our formula X equals negative B over 2A. B in my case is 2 so we're going to make it negative 2 over 2 multiplied by A which in my case is just 1. So this is all going to simplify into just X equals negative 1. Now what that means is you're going to have a vertical line because all X equals are constants, always graph vertical lines. So on a coordinate plane if you go to X equals negative 1, and you go ahead and graph a dotted vertical line there, that's going to be the axis of symmetry of my parabola. So what else do I know about? I know that the vertex is going to fall on this axis and I know that my parabola is going to be facing upwards because my A is positive. So while I don't know yet where the vertex is, I know that it's going to be somewhere along this axis of symmetry and it's going to be facing up. So X equals negative B over 2A is the formula that you're going to need in order to find the axis of symmetry. My name is Marija, and I just showed you how to find the axis of symmetry.


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