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Theorem Double-Angle Formulas
sin 2 2 sin cos
cos 2 cos sin
2
2
cos 2 1 2 sin
2
cos 2 2 cos 1
2
5
If sin , 2 , find the exact value of
13
(a) sin 2
(b) cos 2
a b r
2
2
2
5 b 13
2
2
2
b2 169 25 144
5
13
-12
b 12
b 12
cos
r 13
5
sin
13
12
cos
13
120
5 12
2
13 13
169
sin 2 2 sin cos
2
12 5
cos 2 cos sin
13 13
2
2
144 25 119
169 169 169
2
cos 2 1 2 sin
2
2 sin 1 cos 2
2
1 cos 2
sin
2
2
cos 2 2 cos 1
2
2 cos 1 cos 2
2
1 cos 2
cos
2
2
1 cos 2
sin
2
2
2
1 cos 2
cos
2
1 cos 2
2
tan
1 cos 2
1 cos
sin
2
2
2
2
1 cos
cos
2
2
1 cos
2
tan
2 1 cos
Theorem Half-Angle Formulas
1 cos
sin
2
2
1 cos
cos
2
2
1 cos
tan
2
1 cos
where the + or sign is determined by
the quadrant of the angle 2 .
3
3
If csc , , find the exact value of
2
2
(a) sin
2
(b) cos
2
(c) tan
2
3
so
lies in Quadrant II
2 2 4
2
r
3
csc =
b 2
2
2
2
2
2
2
a b r , so a 2 3
a 945
2
a 5
a 5
cos
r
3
5
5
5
5
1 cos
1 53
3
(a) sin
2
2
6
2
1 53
3
1 cos
(b) cos
6
2
2
2
5
1
3
1 cos
3
(c) tan
3
2
1 cos
1 5
3