Section 5.5. In the previous sections, we used: a) The Fundamental Identities a)Sin²x + Cos²x = 1...
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Transcript of Section 5.5. In the previous sections, we used: a) The Fundamental Identities a)Sin²x + Cos²x = 1...
Double-Angle and Half-Angle Formulas
Section 5.5
In the previous sections, we used:
a) The Fundamental Identitiesa) Sin²x + Cos²x = 1
b) Sum & Difference Formulasa) Cos (u – v) = Cos u Cos v + Sin u Sin v
Now we will use double angle and half angle formulas
Multiple-Angle Formulas
Double-angle formulas are the formulas used most often:
Double-Angle Formulas
u Cosu Sin 2 2u Sin uSin -u Cos 2u Cos 221 -u 2Cos 2
u2Sin - 1 2uTan - 1
u2Tan 2u Tan
2
Use the following triangle to find the following:
Double-Angle Formulas
2
5
Sin 2θ
Cos 2θ
Tan 2θ
29
θ
Use the following triangle to find the following:
Double-Angle Formulas
2
5
Sin 2θ29
= 2Sin θ Cos θ
2
29
2θ
29
5
29
20
Double-Angle Formulas
2
5
Cos 2θ
29
= 2Cos² θ - 1
2
θ
2
29
5
129
252
1-
129
50
29
21
Double-Angle Formulas
2
5
Tan 2θ
29
2Tan - 1
2Tan
θ 2
52
1
52
2
2521
54
254 -15
4
21
20
Use the following triangle to find the following:
Double-Angle Formulas
1
4
Csc 2θ
Sec 2θ
Cot 2θ
17
θ
8
17
15
17
8
15
General guidelines to follow when the double-angle formulas to solve equations:
1) Apply the appropriate double-angle formula
2) Look to factor
3) Solve the equation using the different strategies involved in solving equations
Double-Angle Formulas
Solve the following equation in the interval [0, 2π)
Double-Angle Formulas
Sin 2x – Cos x = 0
1. Apply the double-angle formula2 Sin x Cos x – Cos x = 0
2. Look to factorCos x (2 Sin x – 1) = 0
Double-Angle Formulas
Cos x (2 Sin x – 1) = 0
3. Solve the equation
Cos x = 0 2 Sin x - 1= 0
Sin x = ½ ,
2
2
3x
x ,6
6
5
Solve the following equation in the interval [0, 2π)
Double-Angle Formulas
2 Cos x + Sin 2x = 0
2 Cos x + 2 Sin x Cos x = 0
2 Cos x (1+ Sin x) = 0
2 Cos x = 0 1 + Sin x = 0
Double-Angle Formulas
2 Cos x = 0 1 + Sin x = 0
Cos x = 0 Sin x = -1
,2
2
3x x
2
3
Solve the following equations for x in the interval [0, 2π)
a) Sin 2x Sin x = Cos x
b) Cos 2x + Sin x = 0
Double-Angle Formulas
,2
x ,
2
3,
6
,
6
5,
6
76
11
,2
x ,
6
76
11
Double-Angle Formulas
Sin 2x Sin x = Cos x2 Sin x Cos x Sin x = Cos x
2 Sin²x Cos x – Cos x = 0
Cos x (2 Sin²x – 1) = 0
Cos x = 0 2 Sin²x – 1 = 0Sin²x = ½ Sin x = ± ½
x =
x ,2
,
2
3
,6
,
6
5,
6
76
11
Double-Angle Formulas
Cos 2x + Sin x = 01 – 2Sin² x + Sin x = 0
2Sin² x - Sin x - 1= 0(2 Sin x + 1) (Sin x – 1) = 0
2 Sin x + 1 = 0 Sin x – 1 = 0 Sin x = ½ Sin x = 1
xx = ,2
,
6
76
11
Double-Angle and Half-Angle Formulas
Section 5.5
Evaluating Functions Involving Double Angles
Use the given information to find the following:
Sin 2x Cos 2x Tan 2x
Double-Angle Formulas
13
12 Sin x
x 2
Double-Angle Formulas
13
12 Sin x
x 2
12 13
x-5Sin 2x = 2Sin x Cos x
2
13
12
13
5
169
120-
Double-Angle Formulas
13
12 Sin x
x 2
12 13
x-5Cos 2x = 2Cos² x - 1
2 2
13
5
1
1169
252
1
169
50
169
119-
Double-Angle Formulas
12 13
x-5
Tan 2x
2
5-12
-1
5-12
2
xTan - 1
2Tan x
2
25144 -1
5-24
25
119- 5-
24
119
120
Evaluating Functions Involving Double Angles
Use the given information to find the following:
Sin 2x Cos 2x Tan 2x
Double-Angle Formulas
17
8 x Cos
2 x 2
3
Double-Angle Formulas
17
8 x Cos
2 x 2
3 -15
17
x8
Sin 2x = 2Sin x Cos x
2
17
15-
17
8
289
240-
Double-Angle Formulas
Cos 2x = 2Cos² x - 1
2 2
17
8
1
1289
642
1
289
128
289
161-
15
8 x Cos
2 x 2
3 -15
17
x8
Double-Angle Formulas
2
815-
-1
815-
2
64
225 -18
30-
64161-
830-
161
240
-1517
x8
Tan 2x xTan - 1
2Tan x
2
The next (and final) set of formulas we have are called half-angle formulas.
2
uSin
2
u Cos
2
uTan
2
u Cos1
2
u Cos1
uSin
u Cos - 1
u Cos 1
uSin
The sign of Sin and Cos depend on what quadrant u/2 is in
Use the following triangle to find the six trig functions of θ/2
Half-Angle Formulas
257
θ
2
Sin
2
Cos
2
Tan
10
2
7
1
10
27
Half-Angle Formulas
257
θ
2
Sin
24
2
u Cos1
225
241
225
1
50
1
50
1
25
1
10
2
Half-Angle Formulas
257
θ
2
Cos
24
2
u Cos1
225
241
225
49
50
49
50
7
25
7
10
27
Half-Angle Formulas
257
θ
2
Tan
24
257
25241
25
725
1
7
1
uSin
u Cos - 1
Find the exact value of the Cos 165º.
Half-Angle Formulas
165º is half of what angle?
Cos 165º = 2
330 Cos
2
330 Cos
2
330 Cos1
22
31
22
32
4
32
2
32
Find the exact value of the Sin 105º.
Half-Angle Formulas
105º is half of what angle?
Sin 105º = 2
210Sin
2
210Sin
2
210 Cos12
231
22
32
4
32
2
32
Find the exact value of the Tan 15º.
Half-Angle Formulas
15º is half of what angle?
Tan 15º = 2
30Tan
2
30Tan
30Sin
30 Cos1
21
23 - 1
2
12
32
32
Double-Angle and Half-Angle Formulas
Section 5.5
Half-Angle Formulas:Find , x
2 and
13
12 Sin x Given
2
xTan c)
2
x Cos b)
2
xSin a)
1312
x-5
13
133
13
132
2
3
Half-Angle Formulas
2
xSin a) 1312
x-5
2
u Cos1
213
5-1
213
51
213
18
26
18
13
9
13
3
13
133
Half-Angle Formulas
2
x Cos b) 1312
x-5
a2
u Cos1
213
5-1
213
8
26
8
13
4
13
2
13
132
Half-Angle Formulas
2
xTan c) 1312
x-5
Sin x
xCos 1
1312
135- - 1
13
1213
5 1
1312
1318
12
18
2
3
Half-Angle Formulas:Find ,
2 x 0 and
4
3 Tan x Given
2
xCot c)
2
x Sec b)
2
x Csc a)
4
3
x
510
3
10
3
Half-Angle Formulas
2
xSin a)
2
u Cos1
25
41
25
1
10
1
10
1 10
4
3
x
5
Half-Angle Formulas
2
x Cos b) a
2
u Cos1
25
41
25
9
10
9
10
3
3
10
4
3
x
5
Half-Angle Formulas
2
xTan c)
Sin x
xCos 1
53
54 - 1
5
35
1
3
1 3
4
3
x
5
Solving Equations using the half-angle formulas:
1) Apply the appropriate formula2) Use the various methods we have learned
to solve equations1) Factor2) Combine Like Terms3) Isolate the Trig Function4) Solve the Equation for an Angle(s)
Half-Angle Formulas
Solve the following equation for x in the interval [0, 2π)
Half-Angle Formulas
xCos 2
x Sin 2 2
xCos
2
xCos - 1 2
2
xCos 2
xCos - 1 2
xCos x Cos - 1
x2Cos 1 21 x Cos x ,
3
3
5
Solve the following equation for x in the interval [0, 2π)
Half-Angle Formulas
2
x 2Cos x Sin - 2 22
2
xCos 12 x Sin - 2
2
2
2
xCos 12 xSin - 2 2
Half-Angle Formulas
2
xCos 12 xSin - 2 2
xCos 1 xSin - 2 2 xCos 1 x)Cos - (1 - 2 2
xCos 1 xCos 1 - 2 2 0 xCos -x Cos 2
0 1) - x (Cos x Cos 1 x Cos 0 x Cos
,2
x
,2
3 0
Solve the following equation for x in the interval [0, 2π)
Half-Angle Formulas
0 1 - x Cos 2
xSin
0 1 - x Cos 2
xCos 1
x Cos - 1 2
xCos 1
) (
2
2
Half-Angle Formulas
x Cos - 1 2
xCos 1
) (
2
2
2
xCos1
xCos x 2Cos - 1 2
xCos2 x 4Cos - 2 x Cos - 1 2
0 1 x 3Cos -x Cos2 2 0 1) - x (Cos 1) - x Cos2(
0 1) - x (Cos 1) - x Cos2(
0 1 - x Cos 0 1 - x Cos2
21 xCos 1 xCos
,3
x
3
50 x
Because we squared both sides, check your answers!