Properties and Classification of Triangles

This video tutorial for grade 9 geometry covers the classification of triangles based on angle sizes and side lengths. It explains the properties of scalene, isosceles, and equilateral triangles, and discusses the interior and exterior angles of triangles. The video also includes example problems to demonstrate how to calculate unknown angles using these properties and theorems.

What is the main focus when classifying triangles by angle sizes?

• The difference between angles

• The sum of all angles

• The largest angle

• The smallest angle

Which type of triangle has an angle exactly equal to 90 degrees?

• Acute angled triangle

• Right angled triangle

• Obtuse angled triangle

• Scalene triangle

What theorem is used in right-angled triangles?

• Sine rule

• Angle bisector theorem

• Triangle inequality theorem

• Pythagorean theorem

What is the defining feature of a right-angled triangle?

• One angle is obtuse

• One angle is 90 degrees

• All angles are equal

• Two sides are equal

What is the defining feature of an obtuse-angled triangle?

• One angle is less than 90 degrees

• All angles are equal

• One angle is exactly 90 degrees

• One angle is greater than 90 degrees

What is the defining feature of an acute-angled triangle?

• One angle is greater than 90 degrees

• All angles are equal

• One angle is exactly 90 degrees

• All angles are less than 90 degrees

What is a scalene triangle?

• A triangle with all sides equal

• A triangle with two sides equal

• A triangle with all sides different

• A triangle with a right angle

In an isosceles triangle, what can be said about the angles opposite the equal sides?

• They are different

• They are equal

• They are complementary

• They are supplementary

What is the defining property of an equilateral triangle?

• All sides and angles are equal

• All angles are 90 degrees

• One angle is obtuse

• Two sides are equal

What is the property of angles opposite equal sides in an isosceles triangle?

• They are complementary

• They are equal

• They are supplementary

• They are different