## Math 6 Chapter 2 Lesson 3: Angle measure

## 1. Summary of theory Tóm

### 1.1. Protractors

– Each angle has a measure. The measure of a flat angle is \({180^0}\)

– The measure of each angle does not exceed \({180^0}\)

### 1.2. Compare two angles

We compare two angles by comparing their measures.

– Two angles are congruent if their measures are equal.

**For example:** \(\widehat A = \widehat B\)

– Of two angles, the angle with the greater (smaller) measure is the greater (smaller) angle.

**For example: **\(\widehat A > \widehat B\) or \(\widehat B < \widehat A\)

### 1.3. Right angle. Acute angle. Prison corner. Flat angle

**– Flat angle: **An angle whose measure is \(180^0\).

**– Right angle:** An angle whose measure is \({90^0}\) is a right angle. The measure of a right angle is also denoted by 1v.

**– Prison angle: **An angle greater than a right angle but smaller than a right angle is an obtuse angle.

**– Acute angle:** An angle less than a right angle is an acute angle.

**For example: **The drawing below:

– Flat angle: \(\widehat {MAN}\)

– Right angle: \(\widehat {IBK}\)

– Obtuse angle: \(\widehat {PCQ}\)

– Sharp angle: \(\widehat {EDG}\)

## 2. Illustrated exercise

**Question 1:** Measure the opening of the scissors (h.11) of the compass (h.12)

– The opening of the scissors is 60^{o}

– The aperture of the compass is 50^{o}

**Verse 2:** Figure 16, point I is the midpoint of line segment BC. Measure to check if two angles BAI and IAC are equal?

Use a protractor to find the measure of two angles.

The angle with the larger measure is larger.

Two angles that have the same measure are congruent.

We have:

\(\eqalign{& \widehat {BAI} = {20^o} \cr & \widehat {IAC} = {45^o} \cr} \)

So \(\widehat {BAI} < \widehat {IAC}\).

## 3. Practice

### 3.1. Essay exercises

**Question 1: **Indicate whether each of the following statements is true or false?

a) An angle whose measure \({75^0}\) is an obtuse angle

b) An angle whose measure \({180^0}\) is a right angle

c) An angle that is not an obtuse must be an acute angle

d) An angle less than a right angle must be an obtuse angle

**Verse 2: **For pictures

a) Name the angles of vertex O in the figure.

b) Indicate the measure of the acute angle O, whose side is Ot in that figure.

c) Measure and give the names of the right angles vertex O in the figure.

d) Give the measure of the obtuse angles of vertex O in the figure.

### 3.2. Multiple choice exercises Bài

**Question 1: **Let O be the intersection of the three lines xy, zt, uv. Name the flat angles of vertex O

A. \(\widehat {xOu};\widehat {uOt};\widehat {tOx}\)

B. \(\widehat {xOy};\widehat {uOv};\widehat {zOt}\)

C. \(\widehat {xOy};\widehat {uOv}\)

D. \(\widehat {uOv};\widehat {zOt}\)

**Verse 2: **Given the angles \(\widehat A = {45^0},\widehat B = {98^0},\widehat C = {167^0}\). Choose the wrong statement

A. Angle A is an acute angle

B. \(\widehat A < \widehat B\)

C. Angle B is larger than right angle

D. Angle C is a right angle

**Question 3: **Let n \(\left( {n \ge 2} \right)\) rays have a common origin, of which no two rays overlap. If there are 28 formed angles, what is n?

A. 8

B. 7

C. 6

D. 9

**Question 4: **Given 6 rays with a common origin, the number of angles formed is:

A. 12 angles

B. 15 angle

C. 18 angles

D. 20 angles

**Question 5: **Let \(\widehat {xOm} = {45^0}\) and angle xOm is equal to angle yAn. Then the measure of angle yAn is equal to

A. 50^{0}

B. 40^{0}

C. 45^{0}

D. 30^{0}

## 4. Conclusion

Through this lesson, you should be able to understand the following:

- Recognize that each angle has a definite measure. The measure of a Flat Angle is 180
^{0}. Understanding right angles, right angles, obtuse angles. - Know how to measure angles with a protractor, know how to compare two angles.
- Identify the point in the corner.

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