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Lets verify exterior angle property of a triangle. Class 9 Maths lab manual experiment and activities to conduct.

## Materials Required

1. Cardboard sheet

2. Glazed papers

3. Geometry box

4. White chart paper

5. Tracing paper

6. Adhesive

7. Cutter

### Prerequisite Knowledge

Before verifying the exterior angle property of a triangle pleas go through the topics of Straight angle and Exterior angle property of a triangle

### Theory

For straight line refer to Activity 11.

For exterior angle property of a triangle refer to Activity 12.

### Procedure

1. Take a cardboard sheet of suitable size and by using adhesive, paste a white chart paper on it.

2. Cut out a ΔABC from a glazed paper and paste it on the cardboard as shown in the image below:

3. Now, produce the side BC of ΔABC to the point D; refer figure below

4. Cut out the pair of angles, ∠A and ∠B from the glazed paper by using a tracing paper, as shown below

5. Now, arrange the cut out angles as shown in the below image

### Demonstration

From the last figure above,

∠ACD is an exterior angle.

∠A and ∠B are its two interior opposite angles.

The two cut out angles, ∠A and ∠B together completely cover ∠ACD. So, ∠ACD = ∠A + ∠B.

### Observation

Measure of ∠A = ........., and ∠B = ............, Sum of (∠A + ∠B) = ..........., and measure of ∠ACD = ..........., Therefore, ∠ACD = ∠A + ∠B.

### Result

We have verified the exterior angle property of a triangle.

### Application

Many geometrical problems may be easily done, using this property of a triangle.

### Viva Voce

Question 1: What do you mean by the linear pair axiom?

Answer: If a ray stands on a line, then the sum of the two adjacent angles so formed is 180° and vice-versa. This property is known as linear pair axiom.

Question 2: What would be the measure of exterior angle for each vertex of equilateral triangle?

Answer: 120^{o} (=60^{o}+ 60^{o}).

Question 3: Does an exterior angle of a triangle is smaller than either of its interior opposite angles?

Answer: No, an exterior angle of a triangle is greater than either of its opposite interior angles.

Question 4: If the exterior angle of a triangle is 90^{o}, then what would be the opposite interior angles, supplementary or complementary?

Answer: Complementary.

Question 5: How would you define an exterior angle property of a triangle?

Answer: An exterior angle of a triangle is equal to sum of opposite interior angles.

Question 6: If the sum of interior opposite angles of a triangle is 130^{o}, then what would be the measure of its exterior angle?

Answer: Sum of interior opposite angles = Exterior angle Exterior angle = 130^{o}.

Question 7: What is the angle form by straight line?

Answer: The straight line always form a angle of 180^{o}.

Question 8: What is the condition of exterior angle of a triangle?

Answer: The exterior angle of a triangle is equal to the sum of opposite interior angles.

Question 9: What is the sum of exterior angles of a triangle?

Answer: We know that the sum of exterior angles of a polygon is always 360°. The sum of exterior angles of a triangle is 360^{o}.