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5.2 Functions of Angles and
Fundamental Identities
• To define the six trigonometric functions, start with an
angle in standard position. Choose any point P having
coordinates (x,y) on the terminal side as seen in the figure
below.
r ( x 0) 2 ( y 0) 2
x2 y2
• Notice that r > 0 since distance is never negative.
Copyright © 2007 Pearson Education, Inc.
Slide 8-1
5.2 The Six Trigonometric Functions
• The six trigonometric functions are sine, cosine, tangent,
cotangent, secant, and cosecant.
Trigonometric Functions
Let (x,y) be a point other than the origin on the terminal side
of an angle in standard position. The distance from the
point to the origin is r x 2 y 2 . The six trigonometric
functions of angle are as follows.
y
x
y
tan ( x 0)
cos
sin
x
r
r
x
r
r
csc ( y 0) sec ( x 0) cot ( y 0)
y
x
y
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Slide 8-2
5.2 Finding Function Values of an Angle
Example The terminal side of angle (beta) in
standard position goes through (–3,–4). Find the
values of the six trigonometric functions of .
Solution
r ( 3) 2 ( 4) 2 25 5
4
4
sin
5
5
5
5
csc
4
4
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3
3
cos
5
5
5
5
sec
3
3
4 4
tan
3 3
3 3
cot
4 4
Slide 8-3
5.2 Quadrantal Angles
•
The six trigonometric functions
can be found from any point on
the line. Due to similar triangles,
y y
,
r r
so sin = y/r is the same no matter which point is used to
find it.
Copyright © 2007 Pearson Education, Inc.
Slide 8-4
5.2 Reciprocal Identities
• Since sin = y/r and csc = r/y,
1
1
sin
and csc
.
csc
sin
Similarly, we have the following reciprocal
identities for any angle that does not lead to a
0 denominator.
1
sin
csc
1
csc
sin
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1
cos
sec
1
sec
cos
1
tan
cot
1
cot
tan
Slide 8-5
5.2 Using the Reciprocal Identities
12
Example Find sin if csc =
.
2
1
sin
Solution
csc
1
12
2
2
12
2 1 3
2 3
3
3
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Slide 8-6
5.2 Signs and Ranges of Function Values
•
•
•
•
In the definitions of the trigonometric functions,
the distance r is never negative, so r > 0.
Choose a point (x,y) in quadrant I, then both x
and y will be positive, so the values of the six
trigonometric functions will be positive in
quadrant I.
A point (x,y) in quadrant II has x < 0 and y > 0.
This makes sine and cosecant positive for
quadrant II angles, while the other four functions
take on negative values.
Similar results can be obtained for the other
quadrants.
Copyright © 2007 Pearson Education, Inc.
Slide 8-7
5.2 Signs and Ranges of Function Values
Example Identify the quadrant (or quadrants) of any angle
that satisfies sin > 0, tan < 0.
Solution Since sin > 0 in quadrants I and II, while tan < 0
in quadrants II and IV, both conditions are met only in quadrant II.
Copyright © 2007 Pearson Education, Inc.
Slide 8-8
5.2 Signs and Ranges of Function Values
• The figure shows angle as it increases from 0º to 90º.
• The value y increases as increases, but never exceeds r, so
y r. Dividing both sides by r gives
y
1.
r
• In a similar way, angles in quadrant IV suggests
y
y
1 , so 1 1.
r
r
Copyright © 2007 Pearson Education, Inc.
Slide 8-9
5.2 Signs and Ranges of Function Values
y
• Since sin ,
r
1 sin 1.
for any angle .
• In a similar way,
1 cos 1.
• sec and csc are reciprocals of sin and cos,
respectively, making
sec 1 or sec 1,
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csc 1 or csc 1.
Slide 8-10
5.2 Ranges of Trigonometric Functions
For any angle for which for which the indicated function
exists:
1. –1 sin 1 and –1 cos 1;
2. tan and cot may be equal to any real number;
3. sec –1 or sec 1 and csc –1 or csc 1.
Example Decide whether each statement is possible or
impossible.
(a) sin 8
(b) tan = 110.47
(c) sec = .6
Solution
(a) Not possible since 8 1.
(b) Possible since tangent can take on any value.
(c) Not possible since sec –1 or sec 1.
Copyright © 2007 Pearson Education, Inc.
Slide 8-11
5.2 Pythagorean Identities
• Three new identities from x2 + y2 = r2
– Divide by r2
2
2
x2 y2 r 2
x
y
1
2
2
2
r
r
r
r r
Since cos = x/r and sin = y/r, this result becomes
2
2
2
2
(cos ) (sin ) 1 or sin cos 1.
– Divide by x2
2
x2 y2 r 2
y r
2 2 1
2
x
x
x
x x
2
2
1 tan sec
2
– Dividing by y2 leads to cot2 + 1 = csc2.
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Slide 8-12
5.2 Pythagorean Identities
Pythagorean Identities
sin cos 1 1 tan sec 1 cot csc
2
2
2
2
2
2
Example Find sin and cos, if tan = 4/3 and is in
quadrant III.
Solution
Since is in quadrant III, sin and cos will
both be negative.
2
2
1 tan sec
2
5
3
sec cos
5
3
4
2
1 sec
3
sin 2 1 cos2
2
3
sin 2 1
5
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sin 2
16
4
sin
25
5
Slide 8-13
5.2 Quotient Identities
•
Recall that sin ry and cos rx .Consider the
quotient of sin and cos where cos 0.
sin
cos
•
Similarly
cos
sin
y
r
x
r
y r y
tan
r x x
cot , sin 0.
Quotient Identities
sin
tan
cos
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cos
cot
sin
Slide 8-14