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[Pages:7]Truy cp ti t?i liu hc tp, b?i ging min ph?
PHNG TR?NH THUN NHT I VI sin V? cos
a sin2 x bsin x cos x c cos2 x d1
Cach giai.Xet 2 trng hp : ? Trng hp 1 :Xet cos x 0 sin x 1 .Thay vao (1) xem thoa hay kh?ng thoa.K?t lu?n ? Trng hp 2:Xet cos x 0. Chia hai v? cua (1) cho cos2 x ,r?i a v? phng trinh b?c hai theo tan x ,giai binh thng.
1 a dtan2 x b tan x c d 0.
B?i 1: Gii c?c phng tr?nh sau:
1). 2sin2 x 3 3 sin cos x cos2 x 4 . 2). 3sin2 2x sin 2xcos 2x 4cos2 2x 2 .
3). 2sin2 x 3 3 sin xcos x 3 1 cos2 x 1 .
4). 3sin2 x 4 sin x 8 3 9 cos2 x 0 .
2
2
5). 3 sin2 x 1 3 sin xcos x cos2 x 1 3 0 .
6). 9sin2 x 30sin xcos x 25cos2 x 25 . 7). sin 2x 2sin2 x 2cos 2x . 8). sin2 x sin 2x 2 cos2 x 1 .
2 LI GII
1). 2 sin2 x 3 3 sin cos x cos2 x 4 1
Trng hp 1: cos x 0 sin2 x 1 : 1 2 4 (v? l?).
Trng hp 2: cos x 0 . Chia hai v ca (1) cho cos2 x c
2 sin2 x 3
cos2 x
3 sin cos x cos2 x
cos2 cos2
x x
4 cos2
x
2
tan2
x
3
3 tan x 1 4 1 tan2 x
2 tan2 x 3 3 tan x 5 0 phng tr?nh v? nghim
2). 3sin2 2x sin 2xcos 2x 4cos2 2x 21
Trng hp 1: cos 2x 0 sin2 2x 1 : 1 3 2 (v? l?).
Trng hp 2: cos 2x 0 . Chia hai v ca (1) cho cos2 2x c
3sin2 2x sin 2x cos 2x
cos2 2x
cos2 2x
4 cos2 2x cos2 2x
2 cos2 2x
3 tan2
2x tan 2x 4 2
1 tan2 2x
tan2 x tan 2x 6 0 tan 2x 2 tan 2x 3
tan 2x 2 2x arctan2 k x 1 arctan2 k ,k
2
2
tan 2x 3 2x arctan 3 k x 1 arctan 3 k,k
2
3). 2sin2 x 3 3 sin xcos x 3 1 cos2 x 1
Trng hp 1: cos x 0 sin2 x 1 : 1 2 1 (v? l?).
Trng hp 2: cos x 0 . Chia hai v ca (1) cho cos2 x c:
Truy cp ti t?i liu hc tp, b?i ging min ph?
Truy cp ti t?i liu hc tp, b?i ging min ph?
2 tan2 x 3 3 tan x 3 1 1 tan2 x 3tan2 x 3 3 tan x 3 0
tan
tan
x x
3 3
1
x x
6 4
k k
,k
4). 3sin2 x 4 sin x 8 3 9 cos2 x 0 1
2
2
1 3sin2 x 8sin x cos x 4 3 9 cos2 x 01'
2
22
2
Trng hp 1: cos x 0 sin2 x 1 : 1' 3 0 (v? l?).
2
2
Trng hp 2: cos x 0 . Chia hai v ca (1') cho cos2 x c:
2
2
3 tan2 x 8 tan x 8 3 9 0
2
2
2
' 16 3. 8 3 9 16 24 3 27 3 3 4
tan x 4 3 3 4 3 hoc tan x 4 3 3 4 3 3 8
2
3
2
3
3
tan x 3 x k x 2 k2,k
2
23
3
tan x 3 3 8 x arctan 3 3 8 k x 2arctan 3 3 8 k2,k
23
2
3
3
Vy nghim ca phng tr?nh x 2 k2 , x 2arctan 3 3 8 k2,k
3
3
5). 3 sin2 x 1 3 sin xcos x cos2 x 1 3 0 1
Trng hp 1: cos x 0 sin2 x 1 : 1 1 0 (v? l?).
Trng hp 2: cos x 0 . Chia hai v ca (1) cho cos2 x c:
3 tan2 x 1 3 tan x 1 1 3 1 tan2 x 0 tan2 x 1 3 tan x 3 0
tan x
1
x
k
4 ,k
tan x 3
x
3
k
Nghim ca phng tr?nh ? cho: x k,x k,k
4
3
6). 9sin2 x 30sin xcos x 25cos2 x 251
Trng hp 1: cos x 0 sin2 x 1 : 1 9 25 (v? l?).
Trng hp 2: cos x 0 . Chia hai v ca (1) cho cos2 x c:
9 tan2 x 30 tan x 25 25 1 tan2 x 16 tan2 x 30 tan x 0
Truy cp ti t?i liu hc tp, b?i ging min ph?
Truy cp ti t?i liu hc tp, b?i ging min ph?
tan x 0
x k
tan
x
15 8
x
arctan
15 8
k
,
k
Phng
tr?nh
?
cho
c?
c?c
nghim:
x
k,
x
arctan
15 8
k,k
7). sin 2x 2sin2 x 2cos 2x (1)
1 2 sin xcos x 2 sin2 x 2 cos2 x sin2 x cos2 x sin xcos x 0
cos
x cos
x
sin
x
0
cos cos
x x
0 sin
x
0
cos x 0
2
cos
x
4
0
x x
2 4
k
,k
k
Nghim ca phng tr?nh: x k,x k,k
2
4
8). sin2 x sin 2x 2 cos2 x 1 1
2
1 sin2 x 2 sin x cos x 2 cos2 x 1 1'
2
Trng hp 1: cos x 0 sin2 x 1 : 1' 1 1 (v? l?).
2 Trng hp 2: cos x 0 . Chia hai v ca (1') cho cos2 x c:
tan2 x 2 tan x 2 1 1 tan2 x tan2 x 4 tan x 5 0 tan x 1 tan x 5 2 Vi tan x 1 x k,k
4 Vi tan x 5 x arctan(5) k,k .
Nghim ca phng tr?nh x k , x arctan(5) k,k 4
B?i 2: Gii c?c phng tr?nh sau:
1). 2 sin3 x cos x .
2). 3sin3 x 2sin2 xcos x sin xcos2 x .
3). 6sin x 2cos3 x 5sin 2xcos x .
4). sin x 4sin3 x cos x 0 .
5). 3cos4 x 4sin2 xcos2 x sin4 x 0
6). sin x sin2 x sin x 2cos x 3 1 sin x1 sin x2 .
7). sin3 x cos3 x sin x cos x .
8). 6 sin x 2 cos3 x 5sin 4x cos x . 2 cos 2x
LI GII
1). 2 sin3 x cos x (1)
Trng hp 1: cos x 0 sin x 1 : 1' 1 0 (v? l?).
Trng hp 2: cos x 0 . Chia hai v ca (1') cho cos3 x c:
2.
sin3 cos3
x x
1 cos2
x
2 tan3 x 1 tan2 x 2 tan3 x tan2 x 1 0 tan x 1 x k, k 4
2). 3sin3 x 2sin2 xcos x sin xcos2 x (1)
Truy cp ti t?i liu hc tp, b?i ging min ph?
Truy cp ti t?i liu hc tp, b?i ging min ph?
Trng hp 1: cos x 0 sin x 1 : 1 3 0 (v? l?).
Trng hp 2: cos x 0 . Chia hai v ca (1) cho cos3 x c:
3.
sin3 cos3
x x
2.
sin2 x cos x cos3 x
sin x cos2 cos3 x
x
3 tan3
x
2 tan2
x
tan
x
3 tan3 x 2 tan2 x tan x 0 tan x 0 tan x 1 tan x 1 3
Vi tan x 0 x k,k .
Vi tan x 1 x arctan 1 k, k .
3
3
Vi tan x 1 x k,k . 4
3). 6sin x 2cos3 x 5sin 2xcos x (1)
6sin x 2cos3 x 10sin x cos2 x 1'
Trng hp 1: cos x 0 sin x 1 : 1' 6 0 (v? l?).
Trng hp 2: cos x 0 . Chia hai v ca (1') cho cos3 x c:
6 sin x 2 cos3 x 10 sin x cos2 x 6 tan x 1 tan2 x 2 10 tan x
cos3 x cos3 x
cos3 x
3tan3 x 2 tan x 1 0
tan x 1 x k, k . 4
Vy nghim ca phng tr?nh: x k,k 4
4). sin x 4sin3 x cos x 01
Trng hp 1: cos x 0 sin x 1 : 1 3 0 (v? l?).
Trng hp 2: cos x 0 . Chia hai v ca (1) cho cos3 x c:
sin x 4 sin3 x cos x 0 cos3 x cos3 x cos3 x
tan x 1 tan2 x 4 tan3 x 1 tan2 x 0
3tan3 x tan2 x tan x 1 0 tan x 1 x k, k 4
5). 3cos4 x 4sin2 xcos2 x sin4 x 0
Ta thy cos x 0 kh?ng tha (1), chia hai v (1) cho cos4 x c:
3 4 tan2 x tan4 x 0 tan x 1,tan x 3
Vy nghim ca phng tr?nh l? x k,x kk .
4
3
6). sin x sin2 x sin x 2cos x 3 1 sin x1 sin x2 1
1 sin x1 sin xsin x 2cos x 3 1 sin x1 sin x2
sinx 1 2
1 sin x
1
3 sin2 x 2 sin x cos x
3
0
1
3 sin2 x 2 sin x cos x
3 0
3 Gii (2):
x k2,k 2
Gii (3): Ta thy cos x 0 kh?ng phi l? nghim ca (3), chia hai v (3) cho cos2 x c:
1 3 tan2 x 2 tan x 3 1 tan2 x 0 tan2 x 2 tan x 3 0
Truy cp ti t?i liu hc tp, b?i ging min ph?
Truy cp ti t?i liu hc tp, b?i ging min ph?
7). sin3 x cos3 x sin x cos x 1 Trng hp 1: cos x 0 sin x 1 , 1 1 1(?ng).
Vy x k l? mt h nghim ca phng tr?nh. 2
Trng hp 2: cos x 0 . Chia hai v ca (1) cho cos3 x c:
tan3 x 1 tan x 1 tan2 x 1 tan2 x tan2 x tan x 2 0 phng tr?nh v? nghim.
8). 6 sin x 2 cos3 x 5sin 4x cos x 1
2 cos 2x
iu kin cos 2x 0 2x k x k
2
42
1 6 sin x 2 cos3 x 10 sin 2x cos 2x cos x 6 sin x 2 cos3 x 10 sin x cos2 x 1' .
2 cos 2x
Trng hp 1: cos x 0 sin x 1 , 1' 6 0 (v? l?)
Trng hp 2: cos x 0 . Chia hai v ca (1) cho cos3 x c:
6 tan x 1 tan2 x 2 10 tan x 6 tan3 x 4 tan x 2 0 tan x 1 x k,k 4 So vi iu kin phng tr?nh v? nghim.
Gii c?c phng tr?nh sau:
1). 4 sin3 x cos3 x cos x 3sin x
[D b 1 H B04]
2). 2
2
cos3
x
4
3
cos
x
sin
x
0
[D b 2 H A05]
3). tan x.sin2 x 2sin2 x 3cos 2x sin x.cos x
4).
8
cos3
x
3
cos
3x
5). sin3 x 4sin2 xcos x 5sin xcos2 x 2cos3 x 0
LI GII
1). 4 sin3 x cos3 x cos x 3sin x
Trng hp 1: X?t cos x 0 sin x 1 thay v?o (1) c 4 3 (v? l?).
Trng hp 2: cos x 0 , chia hai v ca (1) cho cos3 x :
4
sin3 cos3
x x
cos3 cos3
x x
cos x cos3 x
3
sin x cos3 x
4
tan3 x 1
1 tan2 x
3 tan x
1 tan2 x
tan3 x tan2 x 3tan x 3 0 tan x 1 tan x 3
Vi tan x 1 tan x tan x k,k
4
4
Vi tan x 3 tan x tan x k,k
3
3
Vi tan x
3
tan
x
tan
3
x
3
k,
k
2). 2
2
cos3
x
4
3
cos
x
sin
x
0
Truy cp ti t?i liu hc tp, b?i ging min ph?
Truy cp ti t?i liu hc tp, b?i ging min ph?
2
cos
x
4
3
3cos x sin x 0 (cos x sin x)3
3cos x sin x 0
(2)
Trng hp 1: X?t cos x 0 sin x 1 , thay v?o (2) c 0 0 (?ng).
Vy cos x 0 x k,k l? mt h nghim ca phng tr?nh.
2
Trng hp 2: X?t cos x 0 , chia hai v ca (2) cho cos3 x :
(cos x sin x)3 cos3 x
3 cos x cos3 x
sin x cos3 x
0
cos x sin x
cos x
3
3. 1 cos2
x
tan x. 1 cos2
x
0
1 tan x3 3 1 tan2 x tan x 1 tan2 x 0 tan x 1 x k,k 4
Kt lun nghim ca phng tr?nh: x k , x k,k
2
4
3). tan x.sin2 x 2sin2 x 3cos 2x sin x.cos x
tan x.sin2 x 2 sin2 x 3 2 cos2 x 1 sin xcos x
tan x.
sin2 cos2
x x
2 sin2 x cos2 x
3
2 cos2 x cos2 x
1 cos2
x
sin x cos x cos2 x
tan x.tan2 x 2 tan2 x 3 2 1 tan2 x tan x
tan3 x 2 tan2 x 6 3 3tan2 x 3tan x tan3 x tan2 x 3tan x 3 0 tan x 3 tan x 1 tan x 3
tan x 3 x k k
3
tan x 1 x k k
4
tan x 3 x k k .
3
4).
8
cos3
x
3
cos
3x
e)
8
cos3
x
3
cos 3x
1
t t x x t
3
3
1
8
cos3
t
cos
3
t
3
8
cos3
t
cos
3t
8
cos3
t
sin
3t
8 cos3 t 3sin t 4 sin3 t
Ta thy cos t 0 kh?ng phi l? nghim ca phng tr?nh
Chia hai v ca cho cos3 t c: 8 3tan t 1 tan2 t 4 tan3 t
8 3tan t 3tan3 t 4 tan3 t tan3 t 3tan t 8 0.
5). sin3 x 4 sin2 xcos x 5sin xcos2 x 2 cos3 x 0 1
Trng hp 1: cos x 0 sin x 1 1 1 0 ( v? l?)
Truy cp ti t?i liu hc tp, b?i ging min ph?
Truy cp ti t?i liu hc tp, b?i ging min ph?
Trng hp 2: cos x 0 , chia hai v ca 1 cho cos3 x c:
sin3 cos3
x x
4
sin2 x cos x cos3 x
5sin x cos2 cos3 x
x
2
cos3 cos3
x x
0
tan3 x 4 tan2 x 5tan x 2 0 tan x 2 tanx = 1
Vi tan x 2 x arctan 2 k k z
Vi tan x 1 x k k z
4
Truy cp ti t?i liu hc tp, b?i ging min ph?
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