Big Ideas Math: Modeling Real Life, Grade 8
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6. Similar Figures
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Exercise 17 Page 82

Practice makes perfect

We are asked to use a ruler to draw two different isosceles triangles that are similar to the given one. Then, we want to find if the ratios of the corresponding heights are equivalent to the ratios of the corresponding side lengths. Let's do these things, one at a time.

Drawing the Triangles

Let's take a look at the given triangle.

Recall that a similarity transformation is a dilation or a sequence of dilations and rigid motions. Let's dilate the given triangle in two different ways to get two different isosceles triangles that are similar to the given one. To dilate a figure with respect to the origin, we multiply the coordinates of each vertex by the scale factor k. (x,y) → (kx,ky) Let's identify the coordinates of the vertices of the given figure. We will also measure the height and sides.

For the first triangle, let's choose k= 2.

(x,y) (2x,2y) Simplify
(0,0) ( 2*0, 2*0) (0,0)
(3,4) ( 2*3, 2*4) (6,8)
(6,0) ( 2*6, 2*0) (12,0)

We can plot the new vertices on the coordinate plane and connect them using a ruler to draw the first triangle. We will also measure its height and sides.

For the second triangle, let's choose k= 0.5.

(x,y) (0.5x,0.5y) Simplify
(0,0) ( 0.5*0, 0.5*0) (0,0)
(3,4) ( 0.5*3, 0.5*4) (1.5,2)
(6,0) ( 0.5*6, 0.5*0) (3,0)

Again, we can plot the new vertices on the coordinate plane, connect them, and measure its height and sides.

Checking the Ratios

Now we will consider the ratios of the original triangle and the new triangles. Let's find the ratios of the corresponding side lengths of the original triangle and the first similar triangle that we drew. The lengths of the sides in the original triangle are 5 and 6. The corresponding side lengths in the first similar triangle are 10 and 12. 5/10 = 6/12 = 1/2 The height of the original triangle is 4 and the height of the first similar triangle is 8. 4/8 = 1/2 We can see that the ratios are equivalent. Now, let's check the ratios for the second similar triangle that we drew. The corresponding side lengths to the sides of measures 5 and 6 are 2.5 and 3. 5/2.5 = 6/3 = 2 The height of the original triangle is 4 and the height of the second similar triangle is 2. 4/2 = 2 We can see that the ratios of the corresponding heights are equivalent to the ratios of the corresponding side lengths.

Now, we want to know if for all similar triangles the ratios of the corresponding heights are equivalent to the ratios of the corresponding side lengths. We drew the similar triangles in Part A by multiplying the coordinate of each vertex of the original figure by the scale factor k.

(x,y) → (kx,ky) This means that also the endpoints of the height follow this dilation and, because of that, the heights are multiplied by the same number as the sides. Therefore, it is true that for all similar triangles the ratios of the corresponding heights are equivalent to the ratios of the corresponding side lengths.