Trig Identities - Camden Central School District
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Transcript Trig Identities - Camden Central School District
Trig Identities
Quotient Identities
sin
cos
cos
cot
sin
tan
Reciprocal Identities
1
sin
csc
cos
1
sec
tan
1
cot
OR
1
csc
sin
OR
sec
1
cos
OR
cot
1
tan
Pythagorean Identities
sin 2 cos2 1 OR
sin 2 1 cos2
1 tan2 sec2 OR
tan2 sec2 1
cot2 1 csc2 OR
cot2 csc2 1
OR
cos2 1 sin 2
Tips for proving trigonometric identities:
1. You want to make the left and right hand sides of the identities
match by substitution and cancellation.
2. Work with the more complicated side of the identity.
3. Begin by writing all expressions in terms of sine and/or
cosine.
4. If there is a squared term, check to see if you can use one of
the Pythagorean identities. If so, use it to replace the squared
term.
5. You are finished when the left hand side of the identity
EXACTLY matches the right side. You can not move a term
from one side to the other side.
Handout
Before we do some identities, lets practice substituting and cancelling.
Write each expression as a single function or a constant.
1. 1 cos2
Hint: look at trig identities!
sin 2
3. tan cot
Hint: change to sin and/or cos.
sin cos
1
cos sin
5. tan csc
1
sin 1
cos sin cos
cos sec
7. cos tan2 1
sec
2
1
1
cos
2
cos cos
sec
Handout
Write each expression as a single function or a constant.
sin 2
2
tan2
cos
1
9. 1
1
sec2
cos2
sin 2 cos2
1
1 sin 2 cos2
2
cos
1
Now we will try some with given ratios.
5
11. If cos and lies in Quadrant II, find
13
the valuesof thefive remainingfive trigonometric
functions.
12
a 2 b2 c 2
sin
13
52 b 2 132
5
25 b 2 169
cos
13
2
13
b
144
12
b 12
12
tan
5
5
13
12
13
sec
5
csc
cot
5
12
Handout
4
and sin 0, find the valuesof theremaining
3
five trigonometric functions.
7
sin
4
3
cos
4
a 2 b2 c 2
32 b 2 42
7
tan
2
9 b 16
3
4
2
b
7
7
b 7
4
7
csc
3
7 7
13. If sec
cot
4
7
4
sec
3
3
cot
7
csc
4 7
7
3
7
3 7
7 7
7
Handout
5
15. If csc and the third quadrant, then cos
4
5
4
3
a 2 b2 c 2
4 2 b 2 52
16 b 2 25
b2 9
b3
4
sin
5
3
cos
5
5
csc
4
Handout
17. If sin .6 and cos 0, thentan
sin
10
6
8
a 2 b2 c 2
62 b2 102
36 b 2 100
b2 64
b8
6
10
tan
6
8
tan .75
Handout
19. If sin
3
and is an acute angle, what is the value of tan cos ?
4
3
4
7
cos
4
3
tan
7
sin
4
3
7
a 2 b2 c 2
32 b 2 42
9 b 2 16
b2 7
b 7
3
7
tan cos
7 4
3
4
Homework
• Handout
#2-20 evens
2. sin csc
1
sin
1
sin
8. sin 2 cot2 cos2
1 cot2
csc2
4. sec2 1
csc x
sec x
6.
tan2
1
sin x 1 cos x cos x
1
sin x 1
sin x
cos x
cot x
10. sin 2 cos tan csc
sin 1
sin cos cos
sin
2
sin 2
7
and cos 0, find
25
the valuesof thefive remainingfive trigonometric
functions.
7
a 2 b2 c 2
sin
2
2
2
25
7 b 25
24
49 b 2 625
cos
25
25
b 2 576
7
b 24
7
tan
24
24
12. If sin
sin 2
tan2 cos2
14.
2
sin 2
sin
1
sin 2
1
1
cos 2
cos2 sin 2
sec2
25
csc
7
25
sec
24
cot
24
7
16. sin 2 cos2 tan2
1 tan2
18. cos k , then thevalueof cos sin cot
sec2
cos sin cos
sin
cos2 k 2
20. Theexpressionsec2 csc2 is equivalent to
1
1
2
2
cos sin
1 sin 2
1 cos2
2 2
2
2
cos sin sin cos
sin 2
cos2
2
2
2
cos sin sin cos2
sin 2 cos2
cos2 sin 2
1
cos 2 sin 2