4.2: Angle Relationships in Triangles
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Transcript 4.2: Angle Relationships in Triangles
Welcome Geometry!
• Take out your homework
• Take out your whiteboard and whiteboard pens.
• Take out a piece of paper and title it:
– 4.2: Angle Relationships in Triangles
Transformation Test
• A: 28-31
• B: 25- 27.5
• C: 22-24.5
Test Corrections
• This will count as a 5 point homework
assignment due TOMORROW
Whiteboards: DO NOW
Classify each triangle by its angles and sides.
1. MNQ
2. NQP
3. MNP
Whiteboards
1. Find the measure of exterior DBA of BCD,
if mDBC = 30°, mC= 70°, and mD = 80°
2. What is the complement of an angle with
measure 17°?
4.2: Angle Relationships in Triangles
• Learning Objective
– SWBAT find the measures and apply theorems of
interior and exterior angles of triangles.
MATH JOKE OF THE DAY
• How many feet are in a yard?
• It depends on how many people are in the
yard!
Materials
Patty paper
Straightedge
Piece of paper split in half
Pencil/eraser
Directions
1. Draw and label triangle ABC on your paper
2. On patty paper, draw a line l and label a point
P on the line
3. Place line l on AB and place point P on angle
B of your triangle. Trace angle B.
Directions
1. Rotate the triangle until point P is on Angle C
and trace angle C. It should be adjacent to
Angle B.
2. Rotate the triangle again and trace angle A
adjacent to angle C.
Answer the following Question on
your Notes:
1. What do you notice about the three angles of
the triangle?
Now do it Again…
• With a different size triangle
• What do you observe about your results?
• Write an equation describing the relationship
among the measures of the interior angles in a
triangle.
• This is the Triangle Sum Theorem
• Tape your triangle and patty paper in your
notebooks.
Triangle Sum Theorem
An auxiliary line is a line that is added to a figure to aid in a
proof.
4
C
Y
2
1
X
5
An auxiliary line used in the
Triangle Sum Theorem
2
A
3
B
Talk with your group…
• What is the relationship between angles 1 and 4?
• What is the relationship between angles 3 and 5?
• Using the angle addition postulate, what do angles 1,
2 and 3 equal?
C
4
2
A
X
1
5
3
B
Proofs are Back!!!!
Example 1: Application
After an accident, the positions of cars are
measured by law enforcement to
investigate the collision.
1. Use the diagram drawn from the
information collected to find mXYZ.
2. Find mYWZ
mXYZ + mYZX + mZXY = 180°
Whiteboards
• Use the diagram
to find mMJK.
A corollary is a theorem whose proof follows directly from
another theorem. Here are two corollaries to the Triangle Sum
Theorem.
Example 2: Finding Angle Measures in
Right Triangles
One of the acute angles in a right triangle
measures 2x°. What is the measure of the other
acute angle?
Let the acute angles be A and B, with mA = 2x°.
mA + mB = 90°
2x + mB = 90
mB = (90 – 2x)°
Whiteboards
• The measure of one of the acute angles in a
right triangle is x°. What is the measure of
the other acute angle?
• Interior
• all points inside the figure
• Exterior
• all points outside the figure.
1.
What are the interior angles?
2.
What are the exterior angles?
Exterior
Interior
Exterior Angle Theorem
The measure of an exterior angle of a
triangle is equal to the sum of the
measures of its non-adjacent interior
angles
4= 1 + 2
Example 3: Applying the Exterior Angle
Theorem
Find mB.
Whiteboards
Find mACD.
Third Angle Theorem
Example 4: Applying the Third Angle
Theorem
Find mK and mJ.
Whiteboards
Find mP and mT.
Whiteboards
1. The measure of one of the acute angles in a right triangle is 56 °. What
is the measure of the other acute angle?
2. Find mABD.
124°
3. Find mN and mP.
75°; 75°
WHITEBOARDS
4. The diagram is a map showing John's house, Kay's house, and
the grocery store. What is the angle the two houses make with the
store?