Phương trình lượng giác 11 ( đầy đủ lí thuyết bài tập )
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sin x<br />
⇔ 5sin − 2 = 3 ⇔ (5sin − 2)(1 + sin ) = 3sin<br />
1+<br />
sin x<br />
2<br />
2<br />
x x x x<br />
⇔ + − =<br />
2<br />
2 sin x 3sin x 2 0<br />
π<br />
4. Điều kiện : cos x ≠ 0 ⇔ x ≠ + kπ .<br />
2<br />
⎡ π<br />
x = + k2π<br />
1 π ⎢<br />
⇔ sin x = = sin ⇔<br />
6<br />
⎢<br />
.<br />
2 6 ⎢ 5π<br />
x = + k2π<br />
⎢⎣ 6<br />
<strong>Phương</strong> <strong>trình</strong><br />
2<br />
⎡ π ⎤ sin x<br />
⇔ ⎢1 − cos( x − ) (1 cos x) 0<br />
2<br />
2<br />
⎥ − + =<br />
⎣<br />
⎦ cos x<br />
2<br />
sin x<br />
⇔ (1− sin x) − (1+ cos x) = 0<br />
2<br />
1−<br />
sin x<br />
2<br />
sin x<br />
⇔ − (1+ cos x) = 0<br />
1+<br />
sin x<br />
⇔ − − + + =<br />
2<br />
(1 cos x) (1 cos x)(1 sin x) 0<br />
⎡ x = k2π<br />
⎡ cos x = 1<br />
⇔ (1− cos x)(cos x − sin x) = 0 ⇔ ⎢<br />
⎢ ⇔ π .<br />
⎣tan x = 1 ⎢x = + kπ<br />
⎢⎣ 4<br />
Ví dụ 6. Giải các phương <strong>trình</strong> sau:<br />
3 3<br />
1. sin x + cos x = sin x − cos x<br />
2.<br />
2<br />
3. sin x + 3 tan x = cos x( 4 sin x − cos x)<br />
3<br />
2 cos x = sin 3<br />
x<br />
1. <strong>Phương</strong> <strong>trình</strong><br />
Lời giải:<br />
⇔ + = − +<br />
3 3 2 2<br />
sin x cos x (sin x cos x)(sin x cos x)<br />
⇔ − + =<br />
3 2 2<br />
2 cos x sin x cos x cos x.sin x 0<br />
2 2<br />
( )<br />
⇔ cos x sin x − sin x cos x + 2 cos x = 0<br />
π<br />
⇔ cos x = 0 ⇔ x = + kπ (Do<br />
2<br />
x − x x + x > ∀x<br />
∈R )<br />
2 2<br />
sin sin cos 2cos 0<br />
3 3<br />
2. <strong>Phương</strong> <strong>trình</strong> ⇔ 2 cos x = 3sin x − 4 sin x<br />
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