Reteaching - Lesson Plans

Name Class Date

Reteaching (continued)

Midsegments of Triangles

Problem AB is a midsegment of GEF. What is the value of x?

2AB = GF 2(2x) = 20

4x = 20 x=5

Exercises

Find the length of the indicated segment.

1. AC 30

2. TU 13

B

R

D 15 E

A

C

U 26 S

QT

2x E

A

B

F

G

20

3. SU 5.3

R

S

T

10.6

U

V

4. MO 4.4

LM N

8.8

O P

5. GH 30

H

G 15

I

F

E

6. JK 9 K

4.5 O J

NL

Algebra In each triangle, AB is a midsegment. Find the value of x.

7.

M

7

A

B

3x

O

N

5x 7

8.

R

3x 15

T

5 A

2x 5

B

S

9. 15x 6.5 L 1.3 K 10x B A

M

10. B

J

3x 11

L 11 2x

A

H

11.

2x 17 10

E x7 B

D

A

F

12.

Q 13.5

Ax B

P

27

R

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Name Class Date

Reteaching

Perpendicular and Angle Bisectors

Perpendicular Bisectors

There are two useful theorems to remember about perpendicular bisectors.

Perpendicular Bisector Theorem

If a point is on the perpendicular bisector of a segment, then it is equidistant from the endpoints of the segment.

Converse of the Perpendicular Bisector Theorem

X is on the perpendicular

bisector, so it is

equidistant from the

3

endpoints A and B.

A

If a point is equidistant from the endpoints of a segment, then it is on the perpendicular bisector of the segment.

Because X is equidistant

from the endpoints C

and D, it is on the

3

perpendicular bisector C of the segment.

X 3 B

X 3 D

Problem

What is the value of x?

Since A is equidistant from the endpoints of the segment, it is on the perpendicular bisector of EG. So, EF = GF and x = 4.

A

7

7

Exercises

Find the value of x.

1. 8

A

M

8

x

B P

4. x 6 D

E

6

2x

G

F

2.

N

2

5

5

L

M

2x P 4

5.

Q

4

S P 12 x 2x R

E x F4 G

3.

5

F 3J 3 G

2x 3

x 2 H

6. R 10x 3

1

U

S

5x 2 T

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Name Class Date

Reteaching (continued)

Perpendicular and Angle Bisectors

Angle Bisectors

There are two useful theorems to remember about angle bisectors.

Angle Bisector Theorem

If a point is on the bisector of an angle, then the point is equidistant from the sides of the angle.

X is on the angle

bisector and is therefore

equidistant from the

B

sides of the angle.

Converse of the Angle Bisector Theorem

If a point in the interior of an angle is equidistant from the sides of an angle, then the point is on the angle bisector.

Because X is in the

interior of the angle

and is equidistant

E

from the sides, X is

on the angle bisector.

A 4X 4

C

D X

F

Problem What is the value of x? Because point A is in the interior of the angle and it is equidistant from the sides of the angle, it is on the bisector of the angle.

BCA ECA x = 40

B

8

C x

A

40

8

E

Exercises

Find the value of x.

7.

70

70 x

8.

4

3x 12

38 38

9. (4x)

10 5

5 (2x 20)

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Reteaching

Bisectors in Triangles

The Circumcenter of a Triangle

If you construct the perpendicular bisectors of all three sides of a triangle, the constructed segments will all intersect at one point.

Circumcenter

This point of concurrency is known as the circumcenter of the triangle.

It is important to note that the circumcenter of a triangle can lie inside, on, or outside the triangle.

The circumcenter is equidistant from the three vertices. Because of this, you can construct a circle centered on the circumcenter that passes through the triangle's vertices. This is called a circumscribed circle.

Problem Find the circumcenter of right ABC. First construct perpendicular bisectors of the two legs, AB and AC. These intersect at (2, 2), the circumcenter. Notice that for a right triangle, the circumcenter is on the hypotenuse.

Exercises

6y

4B

2

2 A 2

D

Cx 246

Coordinate Geometry Find the circumcenter of each right triangle.

1.

6y

4R

2.

y

M4

3.

y

4J

2

2 Q 2

Sx 246

4

O

L 3

x 24

N

H 4 2 O

2

Kx 24

(3, 2)

(1, 1)

Coordinate Geometry Find the circumcenter of ABC.

4. A(0, 0) (5, 4) B(0, 8) C(10, 8)

5. A( - 7, 3) (1, -2) B(9, 3) C(-7, -7)

(0, 0)

6. A( - 5, 2) (-1, 4) B(3, 2) C(3, 6)

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Name Class Date

Reteaching (continued)

Bisectors in Triangles

The Incenter of a Triangle

If you construct angle bisectors at the three vertices of a triangle, the segments will intersect at one point. This point of concurrency where the angle bisectors intersect is known as the incenter of the triangle.

It is important to note that the incenter of a triangle will always lie inside the triangle.

The incenter is equidistant from the sides of the triangle. You can draw a circle centered on the incenter that just touches the three sides of the triangle. This is called an inscribed circle.

Incenter

Problem

Find the value of x.

The angle bisectors intersect at P. The incenter P is equidistant

from the sides, so SP = PT . Therefore, x = 9.

A

Note that PV , the continuation of the angle bisector, is not the correct segment to use for the shortest distance from P to AC.

B

S

x P

9 10

T V

Exercises

Find the value of x.

7.

14

x

14

8. 2x

2 5x 6

C

9.

12

x 6 2x 6

10. 4x

3.5 2x 7

11. 4 x

16

12.

2x 12

4x 8

8

5x

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