# Measurements in circles

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## Concept # Measurements in circles: 1. Radius and Diameter

### Learning objectives

1. Ability to measure radius, diameter, circumference, chord length and angles subtended at the centre and on the circumference of the circle.
2. Radius, diameter and chord lengths are linear measurements.
3. Relate the size of the circle with radius.
4. They realise that to draw a circle knowing the measure of radius or diameter is essential.
5. There can be infinite radii in a circle.
6. Diameter is twice the radius.
7. The students should understand what a chord is.
8. Chords of different lengths can be drawn in a circle.
9. Chord length can be measured using a scale and its units is cm.
10. The length of the chord increases as it moves closer to the diameter.
11. The longest chord in the circle is its diameter.
12. Distance of chord from the centre is its perpendicular distance from the centre.
13. A chord divides the circle into two segments.
14. Angle at the centre of the circle is 360º.
15. Angles in circles are measured using protractor.
16. Circumference and area are calculated using formula.

### Activity No # 1. Measuring radius and diameter.

• Estimated Time: 15 mins
• Materials/ Resources needed:
1. Laptop, goegebra tool, projector and a pointer.
2. students' geometry box
• Prerequisites/Instructions, if any:
1. Circle and its basic parts should have been done.
• Multimedia resources: Laptop
• Website interactives/ links/ / Geogebra Applets : This file was done by ITfC-Edu-Team.

• Process:
1. Initially the teacher can explain the terms: circle, its centre, radius, diameter and circumference.
2. Ask the children “What parameter is needed to draw a circle of required size ?”
3. Show them how to measure radius on the scale accurately using compass.
4. Show them to draw a circle.
5. Given diameter, radius = D/2.
6. Also the other way i.e. If a circle is given, then its radius can be measured by using scale which is the linear distance between centre of the circle and any point on the circumference.
7. To measure diameter, measure the length of that chord which passes through the centre of the circle.

Then she can project the digital tool 'geogebra.' and further clarify concepts.

• Developmental Questions:
1. Name the centre of the circle.
2. Name the point on the circumference of the circle.
3. What is the linesegment AB called ?
4. Name the line passing through the centre of the circle.
5. Using what can you measure the radius and diameter.
6. Name the units of radius/diameter.
• Evaluation:
1. How do you measure exact radius on the compass?
2. Are the children able to corelate the radius/diameter of a circle with its size ?
• Question Corner:
1. If the centre of the circle is not marked , then how do you get the radius for a given circle.
2. How many radii/diameter can be drawn in a circle?
3. Are all radii for a given circle equal ?
4. Is a circle unique for a given radius/diameter ?
5. In how many parts does a diameter divide the circle ? What is each part called ?

### Activity No # 2 Measuring a chord in a circle.

• Estimated Time : 10 minutes
• Materials/ Resources needed:

Laptop, geogebra file, projector and a pointer.

• Prerequisites/Instructions, if any:
• Multimedia resources:

Laptop, geogebra file, projector and a pointer.

• Website interactives/ links/ / Geogebra Applets

• Process:
1. Show the geogebra file and ask the following questions.
• Developmental Questions:
1. The teacher can point to centre of circle and ask the students as to what it is.
2. She can point to radius and ask the students to name it.
3. Then ask them if any two points on the circumference are joined by a line segment what is it called ?
4. How many chords can be drawn in a circle ?
5. Are lengths of all chords the same ?
6. Name the biggest chord in a circle.
7. How do you measure a chord and in what units ?
• Evaluation:
1. Were the students able to distinguish between radius, diameter and chord ?
• Question Corner:

3 After drawing a chord,what are the two segregated parts of the circle called ?

## Concept # 2. Angles in circles

### Learning objectives

1. students should understand that the angle at the centre of the circle is 360 degrees.

### Activity No # 1.The angle at the centre is double the angle at the circumference

• Estimated Time : 40 minutes
• Materials/ Resources needed : Laptop, geogebra file, projector and a pointer.
• Prerequisites/Instructions, if any
1. Circles and its parts should have been done.
• Multimedia resources: Laptop and a projector.
• Website interactives/ links/ / Geogebra Applets

• Process:
1. Project the geogebra file and ask the questions listed below.
• Developmental Questions:
1. Name the centre of the circle?
2. Name the minor arc.
3. Name the point on the circumference of the circle at which the arc subtends an angle.
4. Name all radii from figure.
5. What type of triangle is triangel APO ?
6. Name the two equal sides of the triangle APO.
7. Recall the theorem related to isosceles triangle.
8. Name the two equal angles.
9. Name the exterioe angle for the triangle APO
10. Recall the exterior angle theorem.
11. What relation do you observe between <p and <x.
12. Similarly try to explain the same with triangle PBO.
13. If <APO is half of <AOQ and <BPO is half of <BOQ what can you conclude about angles <AOB and <APB.
14. What relation do you observe between the angle at the centre and that on the circumference formed by the same arc ?
• Evaluation:
1. In a circle, how many angles are subtended by an arc at its centre?
2. In a circle, how many angles are subtended by an arc at its circumference?
• Question Corner:
1. What are the applications of this theorem.

### Activity No # 2. Angles in a circle.

• Estimated Time: 40 minutes
• Materials/ Resources needed:Laptop, projector, geogebra file and a pointer.
• Prerequisites/Instructions, if any
1. Knowledge of a circle, angles, arcs and segments.
2. About the types of angles.
3. Skill of drawing a circle , angles and measuring them.
• Multimedia resources : Laptop, Projector.
• Website interactives/ links/ / Geogebra Applets: This file has been done by Mallikarjun Sudi of Yadgir.

• Process:
1. The teacher can recall the concept of circle, arc segment.
2. She can then project the geogebra file , change slider and make clear the theorems about angles in a circle.

Developmental Questions:

1. Name the minor and major segments.
2. Name the angles formed by them.
3. Where are the two angles subtended ?
4. What is the relation between the two angles.
5. Name the major and minor arcs.
6. What is an acute angle?
7. What is an obtuse angle?
8. What type of angles are formed by minor arc ?
9. What type of angles are formed by major arc ?
10. What are your conclusions ?
• Evaluation:
1. How many angles can a segment subtend on the circumference ?
2. What can you say about these angles ?
• Question Corner:
1. Recall the theorems related to angles in a circle.
• Process:
1. The teacher can recall the concept of circle, arc segment.
2. She can then project the geogebra file , change slider and make clear the theorems about angles in a circle.

Developmental Questions:

1. Name the minor and major segments.
2. Name the angles formed by them.
3. Where are the two angles subtended ?
4. What is the relation between the two angles.
5. Name the major and minor arcs.
6. What is an acute angle?
7. What is an obtuse angle?
8. What type of angles are formed by minor arc ?
9. What type of angles are formed by major arc ?
10. What are your conclusions ?
• Evaluation:
1. How many angles can a segment subtend on the circumference ?
2. What can you say about these angles ?
• Question Corner:
1. Recall the theorems related to angles in a circle.

## Concept # 3. Finding the Circumference of a circle

### Learning objectives

1. The children understand that the distance around the edge of a circle is known as circumference.
2. The children learn to measure the circumference of the circle.
3. Derivation of formula for circumference.
4. They understand what is pi.

### Notes for teachers

The circumference of a circle relates to one of the most important mathematical constants in all of mathematics. This constant pi, is represented by the Greek letter П. The numerical value of π is 3.14159 26535 89793 , and is defined by the ratio of a circle's circumference to its diameter. C = п. D or C = 2пr.

### Activity No # 1 Derivation of formula for circumference and the value for pi.

• Estimated Time : 45 mins
• Materials/ Resources needed:

Note books, compass, pencil, mender, scale.

• Prerequisites/Instructions, if any:
1. Circles basics should have been done.
• Multimedia resources:
• Website interactives/ links/ / Geogebra Applets
• Process:
2. Let them carefully measure their circumferences using wool.
3. Mark the distance around the circle on the wool with a sketch pen.
4. Measure the length of the measured wool using a scale.
5. Make a table with columns radius, diameter and circumference
6. For every circle find Circumference / diameter.
7. Round C/d to two decimal places.
8. Observe the answers in each case. It would be aprroximately 3.14 .
9. The value 3.14 is the value of pi which is constant.

C/d = п or C = п d or C = 2п r.

• Developmental Questions:
1. Have you noted down radius, diameter and their respective circumferences.
2. Check if your calculations are correct.
3. What do you infer from the observed results ?
• Evaluation:
1. Are the children taking correct measurements.
2. Are they comparing the results of C/d with all circles.
3. Are they noticing that it is constant .
4. Are they questioning why it is constant?
• Question Corner:
1. How do you derive the formula for circumference of a circle ?
2. What is the name of that constant ?

## Concept # 4. Finding the area of a circle.

### Learning objectives

1. The child should understand that the area of a circle is the entire planar surface.
2. Derivation of the formula for area of the circle.
3. Area of the circle is dependent on its radius.
4. The formula for area of a circle is derived by converting the circle into an equally sized parallelogram.

### Notes for teachers

1.Proof for area of a circle refer to them following link. http://www.basic-mathematics.com/proof-of-the-area-of-a-circle.html

### Activity No # 1. To discover a formula for the area of a circle.

This activity has been taken from website : http://www.mathsteacher.com.au/year8/ch12_area/07_circle/circle.htm

• Estimated Time:90 mins
• Materials/ Resources needed:A compass, pair of scissors, ruler and protractor , pencil and chart papers.
• Prerequisites/Instructions, if any
1. Prior knowledge of circle, radius and parallelogram area.
2. Skill to measure and draw accurately.
• Multimedia resources
• Website interactives/ links/ / Geogebra Applets
• Process:

Refer this website : http://www.mathsteacher.com.au/year8/ch12_area/07_circle/circle.htm

• Developmental Questions:
1. Calculate the area of the figure in Step 6 by using the formula: Area = base x height
2. What is the area of the circle drawn in Step 1?
3. It appears that there is a formula for calculating the area of a circle. Can you discover it?
• Evaluation:
1. Is the student able to comprehend the idea of area.
2. Is the student able to corelate that the base of the parallelogram formed is half of the circle's circumference.
• Question Corner:
1. What is the area of a parallelogram ?
2. Is there any other way by which you can deduce the formula for area of a circle ?

### Activity No # 2. Proving area of the circle = п r² using geogebra applet.

• Estimated Time: 45mins
• Materials/ Resources needed;

Laptop, geogebra file, projector and a pointer.

• Prerequisites/Instructions, if any:

Prior knowledge of circle, radius, square and area of square.

• Multimedia resources: Laptop.
• Website interactives/ links/ / Geogebra Applets: This file was done by Bindu.

• Process:
1. Show the students the two figures circle and square.
2. Tell them that the radius and side of square are of same measure as it would help us in deducing the formula for area of circle.
3. Formulas are easy ways of calculating area .
4. If formulas are not known then the entire area in question can be divided into small squares of 1 unit measure and can deduce the formula of the whole.
5. First the number of full squares is counted.
6. Then two half squares would add up to 1 full square.
7. Ignore less than quarter . Take 3/4 as full.
8. Finally adding up the whole number would give us the full area of the figure in question.
9. Divide area of circle with that of square and deduce formula for square with known formula for square.
• Developmental Questions:
1. Which are these two figures?
2. What inputs do you need to draw a circle ? And for a square ?
3. What do you observe as constant in the two figures ?
4. Do you think the size of both the figures are same ?
5. How do we find it ?
6. What is the formula to find the area of a square ?
7. When we do not know the formula for area, how do we deduce it ?
8. Count the number of squares in the entire area of circle ?
9. How to add half and quarter squares ?
10. Approximately how many total 1 unit squares cover the circle ?
11. So, what is the area of the circle ?
12. What are we trying to deduce (get) through this activity ?
13. Fine lets try dividing the area of circle with area of square and observe the proceedings while we try to deduce the formula for area of circle.
• Evaluation;
1. Has the student understood the concept of area.
2. Was the student aligned with the assignment and was he able to follow the sequence of steps ?
3. Is the student able to appreciate the analogy ?
• Question Corner;
1. What is Pi ?
2. What do you understand by area ?
3. What is the formula to find the area of square and that of a circle ?

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