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## Lesson 6

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### Lesson 6

Similar and Congruent Triangles

Definition of Similar Triangles

- Two triangles are called similar if they both have the same three angle measurements.
- The two triangles shown are similar.
- Similar triangles have the same shape but possibly different sizes.
- You can think of similar triangles as one triangle being a magnification of the other.

B

A

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Similar Triangle Notation- The two triangles shown are similar because they have the same three angle measures.
- The symbol for similarity is Here we write:
- The order of the letters is important: corresponding letters should name congruent angles.

B

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- Let’s stress the order of the letters again. When we write note that the first letters are A and D, and The second letters are B and E, and The third letters are C and F, and We can also write:

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Proving Triangles Similar- To prove that two triangles are similar you only have to show that two pairs of angles have the same measure.
- In the figure,
- The reason for this is that the unmarked angles are forced to have the same measure because the three angles of any triangle always add up to

When trying to show that two triangles are similar, there are some standard ways of establishing that a pair of angles (one from each triangle) have the same measure:

- They may be given to be congruent.
- They may be vertical angles.
- They may be the same angle (sometimes two triangles share an angle).
- They may be a special pair of angles (like alternate interior angles) related to parallel lines.
- They may be in the same triangle opposite congruent sides.
- There are numerous other ways of establishing a congruent pair of angles.

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Proportions from Similar Triangles- Suppose
- Then the sides of the triangles are proportional, which means:
- Notice that each ratio consists of corresponding segments.

Example

D

- In the figure, are right angles, and Find
- First note that since and since the triangles share angle C.
- Let x denote AB. Then:

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Definition of Congruent Triangles- Two triangles are congruent if one can be placed on top of the other for a perfect match (they have the same size and shape).
- In the figure, is congruent to In symbols:
- Just as with similar triangles, it is important to get the letters in the correct order. For example, since A and D come first, we are saying that when the triangles are made to coincide, A and D will coincide.

Proving Triangles Congruent

- To prove that two triangles are congruent it is only necessary to show that some corresponding parts are congruent.
- For example, suppose that in and in that
- Then intuition tells us that the remaining sides must be congruent, and…
- The triangles themselves must be congruent.

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SAS- In two triangles, if one pair of sides are congruent, another pair of sides are congruent, and the pair of angles in between the pairs of congruent sides are congruent, then the triangles are congruent.
- For example, in the figure, if the corresponding parts are congruent as marked, then
- We cite “Side-Angle-Side (SAS)” as the reason these triangles are congruent.

B

A

F

E

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ASA- In two triangles, if one pair of angles are congruent, another pair of angles are congruent, and the pair of sides in between the pairs of congruent angles are congruent, then the triangles are congruent.
- For example, in the figure, if the corresponding parts are congruent as marked, then
- We cite “angle-side-angle (ASA)” as the reason the triangles are congruent.

B

A

F

E

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AAS- In two triangles, if one pair of angles are congruent, another pair of angles are congruent, and a pair of sides not between the two angles are congruent, then the triangles are congruent.
- For example, in the figure, if the corresponding parts are congruent as marked, then
- We cite “angle-angle-side (AAS)” as the reason the triangles are congruent.

Example

- is isosceles with
- Prove that the angle bisector of bisects
- Draw the angle bisector and let denote the point where it intersects
- We first show that
- We already have one pair of sides congruent and one pair of angles congruent as marked in the figure.
- Note also that (the two triangles share this segment). So, the triangles are congruent by SAS.
- So, by CPCTC.

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Example

- In the figure, a line segment is drawn from the center of the circle to the midpoint of a chord. Prove that this line segment is also perpendicular to the chord.
- First, draw
- Note that because they are both radii.
- Also, (this side is shared by both triangles).
- So, by SSS.
- So, by CPCTC.
- So, since these angles are supplementary they have to each measure

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Example

- In the figure,
- Prove that
- Draw
- Note that because these are alternate interior angles.
- Note that because these are alternate interior angles.
- Note that
- So, by ASA.
- So, by CPCTC.

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