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6. A man starting from a point P walks 60 yd. west and then 35 yd. north. What is his direct distance from P?

Suggestion: On a scale drawing to the right is east, and to the left is west. 7. In building a barn a carpenter aims to make the height of the

roof (Fig. 162) equal to one-fourth of the width of the building. By

means of a scale drawing find the h

angle between the rafter AC and the А. p.....--- -36

B plate AB.

8. Two automobiles leave a garFIG. 162

age at the same time. One running at a rate of 20 miles an hour travels east for two hours, and then north for one hour. The other running at a rate of 25 miles an hour

travels southwest for one hour and north for two hours. Find the distance between them at this time, using a scale drawing.

Suggestions: Make a rough sketch before attempting an accurate drawing. The required line is to be the last line drawn. Southwest means halfway

between south and west С

(p. 89). Fig. 163

9. Solve Exercise 1 (876) by means of a scale draw

ing. The measured parts C с

are given in Fig. 163.

Suggestion: Draw CA first.

10. Solve Exercise 5

(976) by means of a scale 150°

drawing, the measured parts A

B 50 rd.

being shown in Fig. 164. FIG. 164

Suggestion: Draw AB first.

BI

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100 rd.

140 rd.

350

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Check the accuracy of the drawing by finding the third angle.

11. A surveyor wishes to find the distance AB (Fig. 165) between two points, A and B, on opposite sides of a river. B is so located that it cannot be seen from A.

He first draws a straight line DC through A.

He then lays off a distance of 160 ft. from A to C.

36 Placing his transit at C he

200' A

160' measures angle ACB and

Fig. 165 finds it to be 45°.

Similarly, 200 ft. from A he locates D, and finds by measurement that angle ADB=36o. Find the distance from A to B.

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B

81. Angle of elevation. To find the height of a chimney AB (Fig. 166) a surveyor places his transit at a point C taken at a convenient distance from A.

He then points the telescope horizontally in the direction ED.

Turning the telescope through

50 angle DEB, he points it to the top

FIG. 166 of the chimney in the direction EB. Angle DEB is called the angle of elevation of the point B from the point E.

EXERCISES

1. The telescope (Fig. 166) is 4 ft. above the ground and 50 ft. from A. AC is in horizontal position. Measure the angle of elevation, make a scale drawing of AEDB, and from it find the length of DB. Find AB.

2. The angle of elevation of the top of a tree is 30° when observed at a point 40 ft. from the foot of the tree. How high is the tree above the horizontal line of sight?

To test the accuracy of the drawing use $66, Exercise 11, i.e., measure the hypotenuse of the triangle and compare its length with

the length of the side opposite to the 30° angle.

45'

3. A flagpole (Fig. 167) 45 ft. high casts a shadow 55 ft. long. Make a scale drawing and find ECAB. This is the angle of elevation of the sun.

55'

4. When the angle of elevation of the Fig. 167

sun is 30° a building casts a shadow 85 ft. long. Find the height of the building. Can you state a conven

ient way of checking the accuracy of your drawing?

5. On top of a building (Fig. 168) is a tower. At a point A, 75 ft. from the

base of the building, the FIG. 168

angle of elevation of the

top of the tower is 35o, and the angle of elevation of the base of the tower is 20°. Find the height of the tower.

6. To determine the height of a tower AB (Fig. 169) a surveyor

B

places his transit at a point C and finds the angle of elevation EC1B to be equal to 68o.

He next places the transit at a point D in line with C and E and 60 ft. from C. He finds the angle of ele

vation EDB to be 35°. If Der Ci

in both cases the telescope D с

was 3 ft. above the ground, FIG. 169

find the length of AB.

82. Angle of depression. A transit is placed on top of a cliff A (Fig. 170) overlooking a river.

The telescope is first pointed horizontally in the direction AC.

It is then turned through angle CAB until it points

C---

Fig. 170

to a passing boat B. The angle CAB is called the angle of depression of the boat from the point A.

Show that the angle of depression of B from the point A is the same as the angle of elevation of A from the point B.

EXERCISES

1. From the top of a lighthouse 100 ft. high (Fig. 171) the angle of depression of a boat is 50°. How far is the boat from the top of the lighthouse?

30°

100

2. An observation balloon B is anchored 2000 yd. above a point A. The balloonist observes the enemy at a point C and finds the angle of depression of C from B to be 62°. How far is it from A to C?

50°

Fig. 171

3. From the top of a vertical cliff 100 ft. high the angle of depression of a buoy is 30°. Find its distance from the bottom of the cliff.

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83. Advantages of the method. The use of the congruent-triangle method of finding distances is limited because it requires a triangle to be laid off which is congruent with the triangle containing the desired distance as a side. The scale-drawing method is an improvement because the required triangle is drawn to scale on paper, not actually on the ground. However, the errors introduced in making a drawing, and the time spent in attempting to attain a high degree of accuracy in drawing and measuring, offer serious objections to this method. The "method of similar triangles,” which is the next to be studied, has the advantage that an exact drawing is not needed, a rough sketch being sufficient. Furthermore, the final result is not determined by measurement, but by solving an equation, which makes greater accuracy possible.

84. Similar triangles. On squared paper draw a. triangle, as ABC (Fig. 172).

Draw A, B, not equal to AB.

On segment A,Bconstruct triangle A,B,C, so that angle Arangle A1, and angle Brangle Bi.

Tell, without measuring, how angle C should compare with angle C1.

Measure angles C and C, to test the accuracy of the drawing.

How do the two triangles compare as to shape? As to size?

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