Double Angle and Half Angles
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Transcript Double Angle and Half Angles
DOUBLEANGLE AND
HALF-ANGLE
IDENTITIES
If we want to know a formula for sin 2 x we could use
the sum formula.
sin 2 x sin x x sin x co s x co s x sin x
we can trade these places
sin x co s x sin x co s x 2 sin x co s x
This is called the double angle formula for sine since it
tells you the sine of double x
sin 2 x 2 sin x cos x
Let's try the same thing for co s 2 x
co s 2 x co s x x co s x co s x sin x sin x
cos x sin x 1 sin x sin x
2
2
2
1 2 sin x
2
sin x 1 cos x
2
2
2
cos x 1 sin x
2
2
cos x 1 cos x 2 cos x 1
2
2
2
This is the double angle formula for cosine but by
substituting some identities we can express it in a
couple other ways.
cos 2 x cos x sin x
2
2
Double-angle Formula for Tangent
tan 2 x tan x x
tan 2 x
tan x tan x
1 tan x tan x
2 tan x
1 tan x
2
Summary of
Double-Angle Formulas
sin 2 x 2 sin x cos x
cos 2 x cos x sin x
2
2
cos 2 x 1 2 sin x
2
cos 2 x 2 cos x 1
2
tan 2 x
2 tan x
1 tan x
2
sin x
4
5
,
Let's draw a
picture.
x
2
F in d sin 2 x
24
25
sin 2 x 2 sin x cos x
4 3
sin 2 x 2
5 5
4
5
x’
-3
Use triangle to
find values.
x
We can also derive formulas for an angle divided by 2
(called the half angle formula). We’ll do this by using the
double angle formula for cosine that we found.
cos 2 1 2 sin
2
1 cos 2
sin
2
Let’s solve this for sin
sin
2
2
In this formula it is NOT both
+ and - but you must figure
out where the terminal side
of the angle is and put on the
appropriate sign for that
quadrant.
1 cos 2
Now let = x/2
1 co s x
2
sin
x
2
We can also derive a half angle formula for cosine in a
similar manner. We’ll do this by using a different version
of the double angle formula for cosine.
cos 2 2 cos 1
2
1 cos 2
cos
2
Let’s solve this for cos
co s
2
2
In this formula it is NOT both
+ and - but you must figure
out where the terminal side
of the angle is and put on the
appropriate sign for that
quadrant.
1 co s 2
Now let = x/2
1 cos x
2
cos
x
2
Now to derive a half angle formula for tangent, let’s use
the fact that we know that tangent is sine over cosine and
use their half angle formulas.
tan
x
2
sin
co s
x
2
x
tan
2
2
2
x
1 cos 2
1 cos 2
2
1 cos x
1 cos x
Summary
Half-Angle Formulas
sin
cos
tan
x
1 cos x
2
2
x
1 cos x
2
2
x
1 cos x
2
As stated it is NOT both +
and - but you must figure
out where the terminal side
of the angle is and put on
the appropriate sign for
that quadrant.
1 cos x
where the or - is determined by what quadrant
x
2
is in.
You can also derive identities for
x
the half-angle formulas for tan
2
in a couple other forms.
sin x
x 1 cos x
tan
sin x
1 cos x
2
We could find sin 15° using the half angle formula.
sin
30°
1 cos
30°
2
2
Since 15° is half of 30° we could
use this formula if x = 30°
15° is in first quadrant
and sine is positive there
so we want the +
3
1
2
sin 15
2
2
sin 15
2
2
1
3
2
4
3
2
2
3
sin x
4
,
5
Find sin
x
2
x
2
If is in quadrant II
then half would be in
quadrant I where sine is
positive
2 5
5
4
sin
x
2
sin
x
2
1 cos x
2
3
1
5
2
Use triangle to
find cosine
value.
1
2
3
8
5
5
2
1
5
x’
x
-3
4
5
2
5
2
5
5