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Main content
Current time:0:00Total duration:2:38

Video transcript

we're asked to plot the image of quadrilateral ABCD so that's this blue quadrilateral here under a reflection across the x-axis so that's the x-axis and we have our little tool here on Khan Academy where we can construct a quadrilateral and we need to construct the reflection of triangle a b c d and so what we can do is let me scroll down a little bit so we can see the entire coordinate axis we want to find the reflection across the x-axis so i'm gonna reflect point by point and actually let me just move this whole thing down here so that we can so we can see what is going on a little bit clearer so let's just first reflect point let me move this a little bit out of the way so let's first reflect reflect point a so we're going to reflect across the x-axis a is 4 units above the x-axis 1 2 3 4 so it's image a prime we could say it would be 4 units below the x-axis so 1 2 3 4 so let's make this right over here a a prime I'm having trouble putting the C if I move these other characters around ok there you go so there's gonna be my a prime now let me try B B is 2 units above the x-axis so B prime is gonna have the same x-coordinate but it's gonna be 2 units below the x-axis so let's make this our B so this is our B right over here now let's make this our C C right here has the x coordinate of negative 5 and a y-coordinate of negative 4 now C prime would have the same x-coordinate but instead of being 4 units below the x-axis it'll be 4 units above the x-axis so we would have the coordinates negative 5 comma positive 4 so this is going to be our C here so this goes to negative 5 1 2 3 positive 4 and then last but not least D and so let's see D right now is that negative 2 comma negative 1 if we were reflect across the x-axis instead of being one unit below the x-axis will be one unit above the x-axis and we'll keep our x coordinate of negative two and so there you have it we have constructed the action of ABCD across the x-axis and what's interesting about this example is that the original quadrilateral strut is on top of the x-axis so you can kind of see this top part of the quadrilateral gets reflected below it and this bottom part of the quadrilateral gets reflected above it and then you can see that indeed do they they'd indeed do look like reflections flipped over the x-axis