Just change the sign of the -coordinate. Let s look at the triangle from our previous example and reflect

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1 . onstructing Reflections Now we begin to look at transformations that yield congruent images. We ll begin with reflections and then move into a series of transformations. series of transformations applies more than one transformation one at a time to a pre-image. Reflections Reflections are like a mirror image, or flip, of the pre-image. This results in a congruent image, meaning it is not only the same shape but also the same size. t the th grade, we will usually be reflecting across the -axis or the -axis. oth cases need different formulas for the coordinates, so we ll break it down one at a time. Reflecting cross the -xis reflection across the -axis takes points above the -axis to below the -axis and vice versa. It basically flips the shape up and down. To do so, it utilizes these formulas: = = Notice that the -coordinate stays the same in the image as it was in the pre-image. The only real change is that you take the opposite value. So if the -coordinate was positive in the pre-image, it would be negative in the image. If it was negative in the pre-image, it would be positive in the image. asically, if you want to apply a reflection across the -axis, just change the sign of the -coordinates. While the formulas are valid for this reflection, you can imagine how annoying it would be to memorize the formulas for reflections across lots of different lines. Therefore, it is probably much simpler for you to think of a reflection as folding the paper on the line of reflection and stamping the pre-image wherever it lands after the fold. Let s take a triangle with coordinates :,5), :,), :,) and reflect it across the -axis. Since we only need to change the sign of the -coordinates we end up with the image of a triangle with the points :, 5, :,, :, if we use the formulas. If you simply think of folding the paper on the axis, notice that the blue triangle would land exactly where the green one is. It s like the blue triangle is a stamp and stamped down the new green triangle. Reflection across -axis ' ' '

2 Reflecting cross the -xis Seeing how to reflect across the -axis, what do you think will happen if you reflect across the -axis? s expected, it is now the -coordinates that change, and the -coordinates that stay the same. We use these formulas to reflect across the -axis: = = Just change the sign of the -coordinate. Let s look at the triangle from our previous example and reflect it across the -axis. You should verify that it leads to the points :(,5, :(,, :(,. However, notice that it would again be easier to think of folding the paper on the -axis and stamping the green triangle from the blue. Reflection across -axis ' ' ' Reflecting cross Other Lines Since every line has a new formula to reflecting the coordinates, we ll stop looking at those equations. Instead, we ll focus only on the fold and stamp idea. Let s take a triangle and reflect across a couple of different lines to see some examples. We ll begin by reflecting the triangle across the line = which is a horizontal line at a height of as seen to the left. Imagine folding the paper on that red line. Where would the blue triangle land? Stamp a green triangle there in your mind. Did you get the picture to the right? Now let s reflect that same triangle across a diagonal line of =. Fold and stamp and you should get the picture to the left. Fold and stamp! 5

3 Series of Transformations We can also apply multiple transformations to a shape. When we do so, we apply only one transformation at a time IN THE ORDER THEY RE GIVEN. So if we wanted to dilate, translate, and then reflect we would first dilate the pre-image, then translate that new picture, then reflect that translation. For now we ll stick with just the two transformations we have covered so far. Reflect-Reflect Taking a triangle, we can apply a reflection across the -axis and then reflect that across the -axis: ' ' ' ' ' ' Notice that the in-between step is not shown on the final graph. You are welcome to leave the in-between step as you are graphing. Just be sure to label appropriately so we know which is which. lso notice that we now have double prime to represent the second reflection and we end up with the final image points of :, 5, :,, :,. You might notice that this also looks like a rotation by 10 which is true. It turns out that reflecting across both axes gives you a 10 turn. Dilate-Reflect '' '' Let s again take that same triangle and dilate by = with a center of dilation of 0,0) and then reflect across the -axis. Keeping in mind the order, we must dilate first because that was the order listed. Then we ll take that dilated image and reflect it. Take a look. Shrink then fold and stamp that shrunken triangle. '' '' '' Reflection across -axis Reflect that across -axis Final Picture of the Series '' ' ' ' ' '' ' ' '' '' '' '' '' Dilation by = Reflect that across -axis Final Picture of the Series

4 Lesson. Perform the given reflection or series of transformations on each given pre-image. The line of reflection is marked as a red line. When performing a dilation, use the origin as the center of dilation. 1. Reflect across -axis. Reflect across = 3 3. Reflect across =. Reflect across -axis 5. Reflect across =. Reflect across = 7. Reflect across -axis. Reflect across = 9. Reflect across = 7

5 10. Reflect across -axis 11. Reflect across -axis 1. Reflect across -axis 13. Reflect across -axis 1. Reflect across -axis 15. Reflect across -axis 1. Reflect across = 17. Reflect across = 1. Reflect across =

6 19. Reflect across -axis 0. Reflect across -axis 1. Reflect across -axis. Dilate by = 3. Dilate by =. Dilate by = and reflect across -axis and reflect across -axis and reflect across -axis 5. Reflect across -axis. Reflect across -axis 7. Reflect across -axis 9

7 . Reflect the triangle across the line =. Which of the following statements will be true? For each explain why or why not. a. Point is at the same coordinates as Point. b. Point is at the same coordinates as Point. c. Point is at the same coordinates as Point. d. The image s perimeter equals the pre-image s. e. The image s area equals the pre-image s. f. The image is congruent to pre-image. g. Line segment is horizontal. 9. Reflect the triangle across the line =. Which of the following statements will be true? For each explain why or why not. a. Point will be at the same coordinates as Point. b. Point will be at the same coordinates as Point. c. Point will be at the same coordinates as Point. d. The image s perimeter equals the pre-image s. e. The image s area equals the pre-image s. f. The image is similar to pre-image. g. Line segment is vertical. 30. In your own words, explain what a reflection does to a pre-image. Remember to consider the line of reflection in your explanation. 31. How could reflections be used in real life? 50