7.2 Similar Polygons. Geometry Mr. Peebles Spring 2013

Transcription

1 7.2 Similar Polygons Geometry Mr. Peebles Spring 2013

2 Daily Learning Target (DLT) Monday February 25, 2013 I can understand, apply, and remember to identify similar polygons in real-life problems.

3 Geometry Bell Ringer

4 Geometry Bell Ringer Answer: C

5 Identifying similar polygons When there is a correspondence between two polygons such that their corresponding angles are congruent and the lengths of corresponding sides are proportional the two polygons are called similar polygons. Remember that geometric figures that are similar have the same shape and angles but have different sizes. In the next slide, ABCD is similar to EFGH. The symbol ~ is used to indicate similarity. So, ABCD ~ EFGH.

6 B C Similar polygons G F Polygon AB Polygon EF E H A D AB BC CD DA EF FG GH HE

7 Ex. 1: Writing Similarity Statements Pentagons JKLMN and STUVW are similar. List all the pairs of congruent angles. Write the ratios of the corresponding sides in a statement of S proportionality. T J K W U N L V M

8 Ex. 1: Writing Similarity Statements J K S T Because JKLMN ~ STUVW, you can write J S, K T, L U, M V AND N W. U N M L W V You can write the proportionality statement as follows: JK ST KL TU LM UV MN VW NJ WS

9 Ex. 2: Comparing Similar Polygons Decide whether the figures are similar. If they are similar, write a similarity statement. X 6 Y Q 4 R P 8 S Z W 12

10 15 X 6 Y 9 P 10 Q 8 4 R 6 S SOLUTION: As shown, the corresponding angles of WXYZ and PQRS are congruent. Also, the corresponding side lengths are proportional. W 12 Z WX PQ XY QR YZ RS WX PQ So, the two figures are similar and you can write WXYZ ~ PQRS.

11 Ex. 3: Comparing Photographic Enlargements POSTER DESIGN. You have been asked to create a poster to advertise a field trip to see the Liberty Bell. You have a 3.5 inch by 5 inch photo that you want to enlarge. You want the enlargement to be 16 inches wide. How long will it be? Hint you will need to set up a proportion comparing the length and width of the picture and frame.

12 Solution: To find the length of the enlargement, you can compare the enlargement to the original measurements of the photo. 16 in. 3.5 in. x x in. 5 in x 5 x 22.9 inches Trip to Liberty Bell March 24 th, Sign up today! The length of the enlargement will be about 23 inches.

13 Theorem 8.1 Theorem 8.1: If two polygons are similar, then the ratio of their perimeters is equal to the ratios of their corresponding parts. If KLMN ~ PQRS, then P Q KL + LM + MN + NK PQ + QR + RS + SP S K L R KL PQ LM MN QR RS NK SP N M

14 Using similar polygons in real life If two polygons are similar, then the ratio of lengths of two corresponding sides is called the scale factor. In Example 2 on 3 the previous page, the common ratio of 2 is the scale factor of WXYZ to PQRS.

15 Ex. 4: Using similar polygons The rectangular patio around a pool is similar to the pool as shown. Calculate the scale factor of the patio to the pool, and find the ratio of their perimeters. The ratio is the Patio/Pool 32 ft 48 ft 16 ft 24 ft

16 Because the rectangles are similar, the scale factor of the patio to the pool is 48 ft: 32 ft., which is 3:2 in simplified form. The perimeter of the patio is 2(24) + 2(48) 144 feet and the perimeter of the pool is 2(16) + 2(32) 96 feet The ratio of the perimeters is 96, or 2 32 ft 48 ft 16 ft 24 ft

17 NOTE: Notice in Example 4 that the ratio of perimeters is the same as the scale factor of the rectangles. This observation is generalized in the following theorem.

18 Ex. 5: Using Similar Polygons Quadrilateral JKLM is similar to PQRS. Find the value of z. J M 10 K 15 L R 6 Q Z P S Set up a proportion that contains PQ KL QR JK PQ Write the proportion.

19 Ex. 5: Using Similar Polygons Quadrilateral JKLM is similar to PQRS. Find the value of z. J M 10 K 15 L R 6 Q Z P S Set up a proportion that contains PQ KL QR 15 6 JK PQ 10 Z Write the proportion. Substitute Z 4 Cross multiply and divide by 15.

20 Pages 376 (1-16) Assignment

21 Closure On the whiteboards, write down what it means for polygons to be similar.

22 Closure On the whiteboards, write down what it means for polygons to be similar. Answer: Remember that geometric figures that are similar have the same shape and angles but have different sizes.