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Congruent Triangles

Congruent Triangles. Today’s Learning Goals. We will determine three different ways to know if two triangles are identical. We will begin to see identical triangles in different shapes. B. Q. 6 cm. 6 in. 6 in. C. R. 14 cm. D. A. P. S. a). b). Nice…. Definitions.

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Congruent Triangles

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  1. Congruent Triangles

  2. Today’s Learning Goals • We will determine three different ways to know if two triangles are identical. • We will begin to see identical triangles in different shapes.

  3. B Q 6 cm 6 in 6 in C R 14 cm D A P S a) b) Nice… . Definitions • Congruent means that two shapes or objects are exactly the same. The symbol means congruent. • One way to figure out if two objects are congruent is to lay them on top of each other. If they line up perfectly, then they are congruent. • Consider the two pairs of line segments below. Which pair appears to be congruent line segments?

  4. a) b) Congruent Objects • Consider the following two pairs of stars. Which pair of stars appears to be congruent? Great…the stars above b) appear to be congruent. • The line segments and stars were easy to see which were congruent. Often times, we must reason about congruency using the side lengths and angles of the shapes.

  5. 8 5 13 Triangle Review • Previously, we discussed the triangle inequality. In particular, we found out when a triangle could be made given only the three side lengths. • For example, could you make a triangle with the side lengths 5, 8, and 13? Nice…you cannot make a triangle with these three side lengths because 5 + 8 = 13. So the three side lengths form a straight line…not a triangle!

  6. 13 7 25 Triangle Review • Could you make a triangle with the side lengths 25, 13, and 7? Good…you cannot make a triangle with these three side lengths because 13 + 7 < 25. So the three side lengths will never meet to form a triangle!

  7. 12 8 15 Triangle Review • Could you make a triangle with the side lengths 8, 12, and 15? Great…you can make a triangle with these three side lengths because 8 + 12 > 15. • How many triangles can you make with these three given side lengths? Yes…you can only make one triangle with these three side lengths because when these sides come together, the triangle is rigid.

  8. 15 15 12 12 8 12 8 8 8 15 15 12 Congruent Triangles • Consider all of the triangles below with side lengths of 8, 12, and 15. Are these triangles different? No…these triangles are all the same. They are just rotated or flipped in different ways. • So, if all three sides of two triangles are the same, then the two triangles are congruent. This property is known as SSS (side-side-side).

  9. T F 9 9 R P A 4 11 D 14 8 4 6 S 11 6 9 U B C Q E 13 Congruent Triangles • Consider the following two pairs of triangles. Which pair has two congruent triangles? Explain how you know. Nice…PQR  STU by SSS. • SSS is one way to know if two triangles are congruent.

  10. 72 34 13 cm Congruent Triangles • Suppose we were given two angle measurements and the included side length like in the picture below. How many triangles could you make with this information? Yes…only one because the side lengths would be extended until they met in only one place. • If two triangles have two angles and an included side that are the same, then the two triangles are congruent. This property is known as ASA (angle-side-angle).

  11. B E Q T 68 29 68 29 49 49 24 24 U P A C D F R S 13 m 13.2 m 18 ft 18 ft a) b) Congruent Triangles • Consider the following two pairs of triangles. Which pair has two congruent triangles? Excellent…PQR  STU by ASA. • So, SSS and ASA are two ways to know if two triangles are congruent. There is one other way to know if two triangles are congruent.

  12. 8 km 123 21 km Congruent Triangles • Suppose we were given two side lengths and an included angle like in the picture below. How many triangles could you make given this information? Great…only one because there is only one side length that would work to connect the existing sides. • If two triangles have two side lengths and an included angle that are the same, then the two triangles are congruent. This property is known as SAS (side-angle-side).

  13. Congruent Triangles • Consider the following two pairs of triangles. Which pair has two congruent triangles? L F X T 12.5 ft 8 ft 53 53 21 mm 21 mm D 27 16 mm 16 mm 27 W 8 ft Y E 12.5 ft M N S U a) b) Excellent…LMN  TSU and WXY  DEF by SAS. • So, SSS, ASA, and SAS are three different ways to know if two triangles are congruent.

  14. Partner Work • You have 20 minutes to work on the following problems with your partner.

  15. For those that finish early Are the following congruent? Explain why or why not. 78 26 43 78 43 26

  16. Big Idea from Today’s Lesson • SSS, SAS, and ASA are three ways to know that two triangles are congruent.

  17. Homework • Complete Homework Worksheet

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