Phương trình lượng giác 11 ( đầy đủ lí thuyết bài tập )
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A.<br />
⎡ π<br />
⎢x<br />
= + kπ<br />
2<br />
⎢<br />
⎢ 6<br />
x arccos k<br />
⎢⎣<br />
= ± + π 7<br />
B.<br />
⎡ π<br />
⎢x<br />
= + k2π<br />
2<br />
⎢<br />
⎢ 6<br />
x arccos k2<br />
⎢⎣<br />
= ± 7<br />
+ π<br />
C.<br />
⎡ π<br />
⎢x<br />
= + kπ<br />
3<br />
⎢<br />
⎢ 6<br />
x arccos k2<br />
⎢⎣<br />
= ± 7<br />
+ π<br />
D.<br />
⎡ π<br />
⎢x<br />
= + kπ<br />
2<br />
⎢<br />
⎢ 6<br />
x arccos k2<br />
⎢⎣<br />
= ± 7<br />
+ π<br />
<strong>Phương</strong> <strong>trình</strong> đã cho tương đương với<br />
Lời giải:<br />
2 2<br />
3(2cos 2x −1) −(1− cos 2 x) + cos 2x<br />
− 1 = 0<br />
⎡ π<br />
⎡ cos 2x<br />
= −1<br />
⎢x<br />
= + kπ<br />
2<br />
⇔ 7 cos 2x<br />
+ cos 2x<br />
− 6 = 0 ⇔ ⎢<br />
2<br />
⎢<br />
6 ⇔ ⎢<br />
cos 2x<br />
= ⎢ 6<br />
⎢⎣ 7 x = ± arccos + k2π<br />
⎢⎣<br />
7<br />
Bài 87. Giải phương <strong>trình</strong> 4 cos x.cos 2x + 1 = 0<br />
A.<br />
⎡ π<br />
⎢x<br />
= ± + k2π<br />
3<br />
⇔ ⎢<br />
⎢<br />
− 1±<br />
3<br />
⎢x<br />
= ± arccos + k2π<br />
⎣<br />
8<br />
B.<br />
⎡ π<br />
⎢x<br />
= ± + k2π<br />
3<br />
⇔ ⎢<br />
⎢<br />
− 1±<br />
5<br />
⎢x<br />
= ± arccos + k2π<br />
⎣<br />
8<br />
C.<br />
⎡ π<br />
⎢x<br />
= ± + k2π<br />
3<br />
⇔ ⎢<br />
⎢<br />
− 1±<br />
7<br />
⎢x<br />
= ± arccos + k2π<br />
⎣<br />
8<br />
D.<br />
⎡ π<br />
⎢x<br />
= ± + k2π<br />
3<br />
⇔ ⎢<br />
⎢<br />
− 1±<br />
6<br />
⎢x<br />
= ± arccos + k2π<br />
⎣<br />
8<br />
<strong>Phương</strong> <strong>trình</strong><br />
⇔ − + =<br />
2<br />
4 cos x(2 cos x 1) 1 0<br />
Lời giải:<br />
⇔ − + = ⇔ − + − =<br />
⇔<br />
3 2<br />
8cos x 4 cos x 1 0 (2cos x 1)(4cos x 2cos x 1) 0<br />
⎡<br />
⎡ 1 cos x =<br />
cos x =<br />
⎢<br />
2<br />
⎢ 2<br />
⎢<br />
3<br />
2<br />
⇔ ⎢<br />
⇔ ⎢<br />
⎢<br />
2<br />
− 1±<br />
5<br />
4cos x + 2 cos x − 1 = 0<br />
⎢<br />
⎢<br />
⎢ ⎣ ⎢cos<br />
x =<br />
x<br />
⎣<br />
1<br />
8<br />
⎡ π<br />
x = ± + k π<br />
.<br />
− 1±<br />
5<br />
⎢ = ± arccos + k2π<br />
⎣<br />
8<br />
Bài 88. Giải phương <strong>trình</strong><br />
8 8 2<br />
16(sin + cos ) = 17 cos 2<br />
x x x<br />
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