Free Grade 3 Introducing Angle Bisector Common Errors in Examinations Lessons
Free Grade 3 lessons for Introducing Angle Bisector Common Errors in Examinations: 12 real questions with a full answer key and worked solutions. Angles on a straight line add to 180°, around a point to 360°, and inside a triangle to 180°. A polygon with n sides has interior angles adding to (n − 2) × 180°.
Free
Core
25 min
24
Learning objectives
Every objective is checkable, and each one maps to an activity and an assessment.
- UnderstandExplain introducing angle bisector common errors in examinations
Explain introducing angle bisector common errors in examinations accurately in a familiar context.
Assessed by: Exit ticket of four short items
- ApplyCalculate introducing angle bisector common errors in examinations
Calculate introducing angle bisector common errors in examinations accurately in a familiar context.
Assessed by: Exit ticket of four short items
- AnalyseCompare introducing angle bisector common errors in examinations
Compare introducing angle bisector common errors in examinations accurately in a familiar context.
Assessed by: Exit ticket of four short items
- EvaluateJustify introducing angle bisector common errors in examinations
Justify introducing angle bisector common errors in examinations accurately in a familiar context.
Assessed by: Exit ticket of four short items
Prerequisites
Close these gaps first — each has its own free lesson, worksheet and quiz.
How Introducing Angle Bisector Common Errors in Examinations works
Angles on a straight line add to 180°, around a point to 360°, and inside a triangle to 180°. A polygon with n sides has interior angles adding to (n − 2) × 180°.
12 practice questions
Easy to hard, 24 marks in total. Try them first, then open the answers.
- 1.Two angles on a straight line. One is 111°. Find the other.[1]
- 2.Two angles on a straight line. One is 87°. Find the other.[1]
- 3.Two angles on a straight line. One is 23°. Find the other.[1]
- 4.Two angles on a straight line. One is 79°. Find the other.[1]
- 5.A triangle has angles 38° and 54°. Find the third angle.[2]
- 6.A triangle has angles 85° and 21°. Find the third angle.[2]
- 7.A triangle has angles 39° and 21°. Find the third angle.[2]
- 8.A triangle has angles 54° and 34°. Find the third angle.[2]
- 9.Find the size of each interior angle of a regular 8-sided polygon.[3]
- 10.Find the size of each interior angle of a regular 9-sided polygon.[3]
- 11.Find the size of each interior angle of a regular 7-sided polygon.[3]
- 12.Find the size of each interior angle of a regular 10-sided polygon.[3]
Show answers and working
1. 69°
Angles on a straight line add to 180°. 180 − 111 = 69°.
2. 93°
Angles on a straight line add to 180°. 180 − 87 = 93°.
3. 157°
Angles on a straight line add to 180°. 180 − 23 = 157°.
4. 101°
Angles on a straight line add to 180°. 180 − 79 = 101°.
5. 88°
Angles in a triangle add to 180°. 180 − 38 − 54 = 88°.
6. 74°
Angles in a triangle add to 180°. 180 − 85 − 21 = 74°.
7. 120°
Angles in a triangle add to 180°. 180 − 39 − 21 = 120°.
8. 92°
Angles in a triangle add to 180°. 180 − 54 − 34 = 92°.
9. 135°
Sum of interior angles = (8 − 2) × 180 = 1080°. Each angle = 1080 ÷ 8 = 135°.
10. 140°
Sum of interior angles = (9 − 2) × 180 = 1260°. Each angle = 1260 ÷ 9 = 140°.
11. 128.57°
Sum of interior angles = (7 − 2) × 180 = 900°. Each angle = 900 ÷ 7 = 128.57°.
12. 144°
Sum of interior angles = (10 − 2) × 180 = 1440°. Each angle = 1440 ÷ 10 = 144°.
Worked examples
Two angles on a straight line. One is 35°. Find the other.
- Angles on a straight line add to 180°.
- 180 − 35 = 145°.
- Answer: 145°
A triangle has angles 59° and 48°. Find the third angle.
- Angles in a triangle add to 180°.
- 180 − 59 − 48 = 73°.
- Answer: 73°
Find the size of each interior angle of a regular 6-sided polygon.
- Sum of interior angles = (6 − 2) × 180 = 720°.
- Each angle = 720 ÷ 6 = 120°.
- Answer: 120°
Interactive practice
Free, no account required — answer on screen or print it.
Common mistakes
What learners typically get wrong, and how to fix it.
Reads the wrong scale on a protractor.
Correction: Start from the 0 on the arm you are measuring from.
Uses 360° for a triangle.
Correction: Triangle = 180°, quadrilateral = 360°.
Teacher tips
- · Estimate the angle type before measuring.
Parent tips
- · Spot right angles around the house.
Real-life applications
- · Ramps, clock hands, building and design.
Assessment objectives
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- Move on to Introducing Angle Bisector Essentials, Introducing Angle Bisector Techniques, Introducing Angle Bisector Word Problems, which build directly on this idea.
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