## Math 6 Chapter 2 Lesson 7: Practice measuring angles on the ground

## 1. Summary of theory

### 1.1. Angle measuring instrument on the ground

– Measure the angle on the ground with a trigonometer (figure below)

– Structure of the trigonometry:

- A circular disc is placed horizontally on a tripod.
- On the top of the disc, there is a rotating rod that rotates around the center of the disc.
- The two ends of the rod have a vertical plate, each with a gap, two openings with the center of the disc aligned.

### 1.2. How to measure an angle on the ground

Suppose you need to measure the angle ACB on the ground. Proceed by following these steps:

**Step 1:** Place the trigonometer so that the face of the disc is horizontal and its center lies on the vertical line through the vertex C of the angle ACB (when hooking one end of the plumb line to the center of the disc, the plumb end coincides with the point C).

**Step 2: **Return the rod to position \(0^0\) and rotate the plate face to a position so that the pole is driven at A and the two slots are aligned.

**Step 3: **Fix the disc face and bring the rotary rod to a position so that the focus post is at B and the two openings are aligned.

**Step 4:** Read the measure (degrees) of the angle ACB on the plate face. As shown in Figure 42, we read \(\widehat {ACB} = {100^0}\).

## 2. Practice

**Question 1: **What tools are used to measure angles on the ground? Describe the structure of the device.

**Verse 2:** List the steps to measure an angle on the ground.

**Question 3: **Practice measuring the angles of the triangle formed by the three nearest flower beds in front of the classroom.

## 3. Conclusion

Through this lesson, you should know the following:

- Understand the structure and use of the trigonometry.
- Know how to measure angles on the ground.

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