Toán Lớp 11: Giải pt : 2sin3xsinx + 4cos2x + 2 = 0
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Toán Lớp 11: Giải pt : 2sin3xsinx + 4cos2x + 2 = 0
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$ <=> 5cos2x – (2cos^{2}2x – 1) + 2 = 0$
$ <=> 2cos^{2}2x – 5cos2x – 3 = 0$
$ <=> (2cos2x + 1)(cos2x – 3) = 0$
$ <=> 2cos2x + 1 = 0$
$ <=> cos2x = – \dfrac{1}{2} = cos120^{0}$
– TH1 $: 2x = 120^{0} + k.360^{0} <=> x = 60^{0} + k.180^{0}$
– TH1 $: 2x = – 120^{0} + k.360^{0} <=> x = – 60^{0} + k.180^{0}$