# Relationship between perimeters of similar triangles definition

### geometry - Relationship between Perimeter of 2 Similar Triangle - Mathematics Stack Exchange

Students need to know how to determine if figures are similar. Have students use the definition of similarity to verify that the three rectangles are similar to each the area and perimeter of figures will change in relation to their scale factors;. Two figures that have the same shape are said to be similar. When two figures are similar, the ratios of the lengths of their corresponding sides are equal. When two figures are similar, the ratios of the lengths of their corresponding sides are equal. To determine if the triangles shown are similar, compare their corresponding sides. Are these ratios First Glance, In Depth, Examples, Workout.

If not, ask students to rework the rule and try again. Emphasize that the rule will only be useful if it works for all figures, all the time! In this step, students are just putting the last statement completed on the triangle log sheet into their own words. What will the teacher do to bring the lesson to a close?

How will the students make sense of the investigation? Teacher returns to the similar rectangles question from the "hook. Tell them to write their explanation in terms of the scale factor of the rectangles. The teacher should be sure that all students realize that if two 2D figures have a scale factor ofthen their perimeters will differ by a factor ofand their areas will differ by a factor of.

Summative Assessment Teacher will draw a new set of similar rectangles and triangles on the board. Ask students to identify the scale factors that relate the similar rectangles and the scale factor that relates the similar triangles. Next, ask for volunteers to predict how the area and perimeter of figures will change in relation to their scale factors; e. Have them justify that the triangles they have drawn are indeed similar.

Then have them identify the scale factor that relates the two triangles. Formative Assessment Examine the four triangles in the figure below. Downloadable attachment Measure all three sides and all three angles of each triangle.

Make three observations about the measures you found. What is different about certain measures in the third triangle? Teacher looks for observations that capture the following ideas: The two triangles in the first row have corresponding side lengths that are the same, and their corresponding angles are the same.

The triangle in the second row has sides that are twice as long as the corresponding sides of either triangle in the first row, but corresponding angles are unchanged. The angles of the triangle in the third row are different from the corresponding angles of triangles in the first two rows. In addition, its proportions are different from the triangles in the first two rows.

## Similar Triangles: Perimeters and Areas

Teacher elicits the definitions of similarity and congruence. Feedback to Students Students will receive immediate feedback when answering questions.

The teacher will look for specific areas of difficulty such as: Students may not realize that similarities preserve shape, but not size. Angle measures stay the same, but side lengths change by a constant scale factor. Students may incorrectly apply the scale factor. For example, students will multiply instead of divide with a scale factor. Students will receive further feedback when they submit their independently produced work the optional homework assignment.

Teacher will draw a new set of similar rectangles and triangles on the board. Large diagrams of each step can be provided. Teacher needs to read all instructions and repeat several times.

Ask students to determine the rule that relates scale factor to the volumes of 3D solids that are similar. For example, if one side of a rectangular solid is 2, and the corresponding side of a similar rectangular solid is 3, then the ratio of their volumes is 8: Emphasize that scale factor multiplies any linear dimension of any 2D object.

For example, if a the length and width of a rectangle are doubled in size, then the diagonal measurement a linear dimension also doubles in size. If a circle has radius one-third the radius of a larger circle, then its circumference and diameter both linear dimensions are also one-third the length of the corresponding value of the larger circle.

Relate the results of this lesson to real-life. For example, doubling the length and width of a home causes floor areas to increase by a factor of four.

If the smaller home had 1, square feet, the larger home would have 4, square feet. Since homes are often priced by how many square feet they have, the larger home would be roughly four times as expensive. Not two times, as you might have thought before doing this lesson! Another example might pertain to television sets. We multiplied by 3 there.

### Triangles (Geometry, Similarity) – Mathplanet

To go from the length of XY to the length of AB, which is the corresponding side, we are multiplying by 3. We have to multiply by 3. And then to go from the length of YZ to the length of BC, we also multiplied by 3. If they were the same scale, they would be the exact same triangles.

## Intro to triangle similarity

But one is just a bigger, a blown-up version of the other one. Or this is a miniaturized version of that one over there. If you just multiply all the sides by 3, you get to this triangle.

And so we can't call them congruent, but this does seem to be a bit of a special relationship. So we call this special relationship similarity.

**Geometry 7.5b, Proportional Perimeters and Areas Theorem**

And so, based on what we just saw, there's actually kind of three ideas here. And they're all equivalent ways of thinking about similarity. One way to think about it is that one is a scaled-up version of the other. So scaled-up or -down version of the other.

When we talked about congruency, they had to be exactly the same. You could rotate it, you could shift it, you could flip it. But when you do all of those things, they would have to essentially be identical. With similarity, you can rotate it, you can shift it, you can flip it. And you can also scale it up and down in order for something to be similar. They are scaled up by a factor of 1. But we can't say it the other way around. And we see, for this particular example, they definitely are not congruent.

So this is one way to think about similarity. The other way to think about similarity is that all of the corresponding angles will be equal. So if something is similar, then all of the corresponding angles are going to be congruent.

I always have trouble spelling this. Corresponding angles are congruent. So if you have two triangles, all of their angles are the same, then you could say that they're similar. Or if you find two triangles and you're told that they are similar triangles, then you know that all of their corresponding angles are the same. And the last way to think about it is that the sides are all just scaled-up versions of each other.

So the sides scaled by the same factor. In the example we did here, the scaling factor was 3.