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Chapter 21

Introduction to trigonometry
21
...

The theorem of Pythagoras and trigonometric ratios
are used with right-angled triangles only
...

In this chapter, three trigonometric ratios – i
...
sine,
cosine and tangent – are defined and then evaluated
using a calculator
...


21
...


From equation (1):
b = a2 + c2
Transposing√equation (1) for a gives a 2 = b2 − c2 , from
which a = b2 − c 2
2
2
2
Transposing
√equation (1) for c gives c = b − a , from
2
2
which c = b − a
Here are some worked problems to demonstrate the
theorem of Pythagoras
...
In Figure 21
...

In the right-angled triangle ABC shown in Figure 21
...
1
DOI: 10
...
00021-1

a

c 5 3 cm

B

Figure 21
...
e
...


A
c

A

a

C

Hence,
√
25 = ±5 but in a practical example like this an answer
of a = −5 cm has no meaning, so we take only the
positive answer
...


182 Basic Engineering Mathematics
PQR is a 3, 4, 5 triangle
...
e
...

Problem 2
...
3, find the length of EF

From Pythagoras’ theorem,
BC 2 = 12002 + 8802
= 1440000 + 774400 = 2214400
√
BC = 2214400 = 1488 km
...


D
e5 13 cm

f 5 5 cm
E

Now try the following Practice Exercise

F

d

Figure 21
...


Find the length of side x in Figure 21
...


2

41 cm

169 = d 2 + 25

x

d 2 = 169 − 25 = 144
√
d = 144 = 12 cm

Thus,

40 cm

d = EF = 12 cm

i
...


DEF is a 5, 12, 13 triangle, another right-angled
triangle which has integer values for all three sides
...
5

2
...
6(a)
...


Find the length of side x in Figure 21
...


Problem 3
...
One travels due north at an average
speed of 300 km/h and the other due west at an
average speed of 220 km/h
...
7 mm

as shown in Figure 21
...
The distance apart after
4 hours = BC
...
6

E
S

C

Figure 21
...
3 mm
(b)

B

1200 km

880 km

A

4
...
Determine the length of
AC, correct to 2 decimal places
...


A tent peg is 4
...
0 m high
tent
...


In a triangle ABC, ∠B is a right angle,
AB = 6
...
78 cm
...


14
...
8 shows a cross-section of a component that is to be made from a round bar
...

x

90◦ ,

7
...
83 mm and CE = 28
...

Determine the length of DE
...


Show that if a triangle has sides of 8, 15 and
17 cm it is right-angled
...


183

Triangle PQR is isosceles, Q being a right
angle
...
46 cm find (a)
the lengths of sides PQ and QR and (b) the
value of ∠QPR
...
A man cycles 24 km due south and then 20 km
due east
...
Find the distance
between the two men
...
A ladder 3
...
0 m from the
wall
...
8

21
...
9,
opposite side
hypotenuse

sine θ =

‘Sine’ is abbreviated to ‘sin’, thus sin θ =

BC
AC
C

12
...
One
travels due west at 18
...
6 knots
...

13
...
7 shows a bolt rounded off at one
end
...


m

4m

␾7

se

nu

te
po

Hy

Opposite

␪
A

Adjacent

B

Figure 21
...
Remembering these three equations
is very important and the mnemonic ‘SOH CAH TOA’
is one way of remembering them
...
7

184 Basic Engineering Mathematics
SOH indicates sin = opposite ÷ hypotenuse

sin C =

CAH indicates cos = adjacent ÷ hypotenuse
TOA indicates tan = opposite ÷ adjacent

cos C =

Here are some worked problems to help familiarize
ourselves with trigonometric ratios
...
In triangle PQR shown in
Figure 21
...
10

sin θ =

opposite side
PQ
5
=
=
= 0
...
9231
hypotenuse
PR
13

tanθ =

opposite side
PQ
5
=
=
= 0
...
In triangle ABC of Figure 21
...
47
= 0
...
778
4
...
7996
5
...
47
=
= 0
...
62
4
...
7996
5
...
47
=
= 0
...
778
4
...
3314
3
...
If tan B = , determine the value of
15
sin B, cos B, sin A and tan A
A right-angled triangle ABC is shown in Figure 21
...

8
If tan B = , then AC = 8 and BC = 15
...
12

i
...

from which

AB 2 = 82 + 152

AB = 82 + 152 = 17
AC
AB
BC
cos B =
AB
BC
sinA =
AB
BC
tanA =
AC
sin B =

3
...
62 cm

C

Figure 21
...
e
...
472 + 4
...
472 + 4
...
778 cm

from which

AB
AC
BC
AC
AB
BC
BC
AC
AB
AC
BC
AB

By Pythagoras, AB 2 = AC 2 + BC 2

A

B

opposite side
=
hypotenuse
adjacent side
=
hypotenuse
opposite side
=
adjacent side
opposite side
=
hypotenuse
adjacent side
=
hypotenuse
opposite side
=
adjacent side

8
17
15
=
17
15
=
17
15
=
8
=

or 0
...
8824
or 0
...
8750

Problem 7
...
Determine (a) the distance AB and
(b) the gradient of the straight line AB

Introduction to trigonometry
f (x)
8
7
6

f(x)
8
7
B
6

4
3
2

4
3
2

0

A

2

4
(a)

6

B

␪

0

8

C

A

2

4
(b)

6

15
, find sin X and cos X , in frac4
...

5
...
15, find (a) sin α (b) cos θ (c) tan θ
...
13

185

␣

17
␪
15

Figure 21
...
13(a)
...
13(b), the horizontal and vertical
lines AC and BC are constructed
...
(a)

Exercise 139 (page 328)
1
...
(a) −6 cos x + c
2
3 2x
12
...
(a)

355

4t 3
1
+c
(b) − + 4t +
t
3

Exercise 140 (page 330)

θ3
3

+c

1
...
5 (b) 0
...
(a) 105 (b) −0
...
(a) 6 (b) −1
...
(a) −0
...
833

5
...
67 (b) 0
...
(a) 0 (b) 4

7
...
248

8
...
2352 (b) 2
...
(a) 19
...
457

10
...
2703 (b) 9
...
proof
5
...
5
8
...
67

2
...
7
...
2
...
32
7
...
140 m

4
...
33 Nm

Index
Acute angle, 165
Acute angled triangle, 171
Adding waveforms, 278
Addition in algebra, 62
Addition law of probability, 307
Addition of fractions, 10
numbers, 1, 18
two periodic functions, 278
vectors, 267
by calculation, 270
Algebra, 61, 68
Algebraic equation, 61, 73
expression, 73
Alternate angles, 165, 191
Ambiguous case, 207
Amplitude, 199
Angle, 165
Angle, lagging and leading, 200
types and properties of, 165
Angles of any magnitude, 196
depression, 191
elevation, 191
Angular measurement, 165
velocity, 202
Annulus, 226
Arbitrary constant of integration, 325
Arc, 231
Arc length, 233
Area, 219
Area of common shapes, 219, 221
under a curve, 330
Area of circle, 222, 233
common shapes, 219
irregular figures, 257
sector, 222, 233
similar shapes, 229
triangles, 205
Arithmetic, basic, 1
Average, 299
value of waveform, 260
Axes, 130
Bar charts, 289
Base, 47
Basic algebraic operations, 61
BODMAS with algebra, 71
fractions, 13
numbers, 6

Boyle’s law, 46
Brackets, 6, 68
Calculation of resultant phasors, 281,
283
Calculations, 22, 28
Calculator, 22
addition, subtraction, multiplication
and division, 22
fractions, 26
π and e x functions, 28, 118
reciprocal and power functions, 24
roots and ×10 x functions, 25
square and cube functions, 23
trigonometric functions, 27
Calculus, 313
Cancelling, 10
Cartesian axes, 131
co-ordinates, 214
Charles’s law, 42, 142
Chord, 230
Circle, 222, 230, 233
equation of, 236
properties of, 230
Circumference, 230
Classes, 293
Class interval, 293
limits, 295
mid-point, 293, 295
Coefficient of proportionality, 45
Combination of two periodic functions,
278
Common factors, 69
logarithms, 111
prefixes, 53
shapes, 219
Complementary angles, 165
Completing the square, 105
Cone, 245
frustum of, 252
Congruent triangles, 175
Construction of triangles, 179
Continuous data, 288
Co-ordinates, 130, 131
Corresponding angles, 165
Cosine, 27, 183
graph of, 195
Cosine rule, 205, 281
wave, 195

Cross-multiplication, 75
Cube root, 23
Cubic equation, 161
graphs, 161
units, 240
Cuboid, 240
Cumulative frequency distribution,
293, 297
curve, 293
Cycle, 199
Cylinder, 241
Deciles, 304
Decimal fraction, 216
places, 13, 18
Decimals, 16
addition and subtraction, 19
multiplication and division, 19
Definite integrals, 328
Degrees, 27, 165, 166, 232
Denominator, 9
Dependent event, 307
Depression, angle of, 191
Derivatives, 315
standard list, 321
Derived units, 53
Determination of law, 147
involving logarithms, 150
Diameter, 230
Difference of two squares, 103
Differential calculus, 313
coefficient, 315
Differentiation, 313, 315
from first principles, 315
of ax n , 315
of e ax and ln ax, 320
of sine and cosine functions, 318
successive, 322
Direct proportion, 40, 42
Discrete data, 288
standard deviation, 302
Dividend, 63
Division in algebra, 62
Division of fractions, 12
numbers, 3, 4, 19
Divisor, 63
Drawing vectors, 266

358 Index
Parabola, 156
Parallel lines, 165
Parallelogram, 219
method, 267
Peak value, 199
Pentagon, 219
Percentage component bar chart, 289
error, 36
relative frequency, 289
Percentages, 33
Percentile, 304
Perfect square, 105
Perimeter, 171
Period, 199
Periodic function, 200
plotting, 238
Periodic time, 200
Phasor, 280
Pictograms, 289
Pie diagram, 289
Planimeter, 257
Plotting periodic functions, 238
Polar co-ordinates, 214
Pol/Rec function on calculator, 217
Polygon, 210
frequency, 293, 296
Population, 289
Power, 47
series for e x , 119
Powers and roots, 47
Practical

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