Triangle Angle Sum and Exterior Angles

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Mathematics · Geometry
SP-GEOM-002 · Free to print and use in your classroom · substituteplans.com
Page 2
Triangle Angle Sum and Exterior Angles
Mathematics · Geometry · Grades 9-10 · 45 minutes
Time needed
45 minutes (one class period)
Prep
Copy the 2-page worksheet (one per student). Write the three triangle facts on the board.
Technology
None. A basic calculator is optional.
Student
independence
High. Students work alone with a short partner check at the end.
Copies to make
One 2-page worksheet per student
Materials
Pencils, Worksheet, Whiteboard or chalkboard
Goal for today: Students will apply the triangle angle sum and exterior angle theorems to find unknown angles and
will classify triangles by angles and sides.
Before the bell
☐ Copy the worksheet, one per student (2 pages).
☐ Write on the board: 1) The three angles of a triangle add up to 180°. 2) An exterior angle equals the sum of
the two interior angles that are NOT next to it. 3) Equal sides have equal angles across from them.
☐ Keep the answer key on your desk. You do not need to solve anything yourself.
Say this to start
Good morning! Today’s geometry practice is about triangles. All you need to remember is that the three inside angles
of any triangle add up to 180 degrees. Your students will use that fact, plus one more rule on the board, to find missing
angles. Please keep the room calm and let students work on their own at first.
Run of show
Minutes
What you do
What students do
0–5
Take attendance. Hand out the worksheet face down.
Settle in and take out a pencil.
5–12
Read the three facts on the board aloud. Then read this
example: A triangle has angles 50° and 60°. The third
angle is 180 − 50 − 60 = 70°.
Follow along and copy the facts into the
margin if they like.
12–20
Tell students to do Part A (questions 1–3), the fill-in facts,
then start Part B.
Fill in the blanks and begin the angle
problems.
20–34
Tell students to finish Part B (questions 4–10). Walk the
room. If a student is stuck on 9 or 10, ask: “Which angle is
the outside angle, and which two angles are far away from
it?”
Write an equation for each problem and
solve it.
34–41
Tell students to answer questions 11–12 and fill in the
Classify table. Partners may compare quietly.
Choose answers and classify each triangle.
41–45
Read the answers aloud from the key while students check.
Collect all worksheets.
Check their work and hand in the
worksheet.

SubstitutePlans.com
Mathematics · Geometry
SP-GEOM-002 · Free to print and use in your classroom · substituteplans.com
Page 3
If things go wrong
• If a student says, “I don’t know what an exterior angle is,” explain: it is the angle formed outside the triangle
when you extend one side in a straight line.
• If students want a diagram, tell them to sketch a quick triangle and label the angles from the words.
Sketching is encouraged.
• If a student gets a total that is not 180°, tell them to add their three angles to check.
• If someone finishes very quickly, ask them to check every answer by adding the three angles, then use the
early finisher list.
Early finishers
• Draw a triangle with angles of 90°, 45° and 45°. Label its sides as equal or not equal.
• Explain in one sentence why a triangle can have at most one obtuse angle.
• Make up your own exterior-angle problem and trade with a neighbor.
Support and challenge
• Support: Allow calculators and have students complete questions 1–7 and the table only.
• Challenge: Ask students to find all three angles of a triangle whose angles are in the ratio 2 : 3 : 4.
Notes for the teacher (you fill this in)
Jot a few lines before you leave. A full Daily Sub Report is in the free Sub Survival Kit at SubstitutePlans.com.
How far the class got / what to pick up next:
Students who were especially helpful:
Anything the teacher should know: