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3sin5x = 5sin3x
<=> - 3sin5x + 5sin3x = 0
<=> - 3sin5x + 3sin3x + 2sin3x = 0
<=> 3(sin3x - sin5x) + 2sin3x = 0
<=> -6cos4x.sinx + 2(3sinx - 4sin³x) = 0
<=> sinx [ -6cos4x + 6 - 8sin²x ] = 0
<=> 2sinx [ -3cos4x + 3 - 4sin²x ] = 0
*Trường hợp sinx = 0
=>x = kπ (k = 0 , ± 1 , ±2 ,...)
*Trường hợp : -3cos4x + 3 - 4sin²x = 0
=> -3(2cos²2x - 1) + 3 + 2(cos2x - 1) = 0
=> -6cos²2x + 2cos2x + 4 = 0
=> cos2x = 1 hoặc cos2x = -1/3
+ cos2x = 1 = > 2x = k2π => x = kπ
+ cos2x = -1/3 = cosα (α = arccos(-1/3) )
=> 2x = ± α + k2π => x = ± (α /2) + kπ
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