luong giac

  • Thread starter dandelion_namduong94
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hocmai.toanhoc

Chào em!
Hocmai giúp em tiếp bài 1 nhé!
pt <=> 5cos(3x+pi/2) + 3cos(5x-pi/2) = 0
<=> 5sin(-3x) + 3sin5x = 0 <=> 3sin5x - 3sin3x - 2sin3x = 0
<=> 3(sin5x - sin3x) - 2sin3x = 0
<=> 3.2.cos4x.sinx - 2(3sinx - 4sin³x) = 0
<=> 3cos4x.sinx - 3sinx + 4sin³x = 0
<=> 3(cos4x -1).sinx + 4sin³x = 0
<=> 3.(-2sin²2x).sinx + 4sin³x = 0
<=> -24.sin³x.cos²x + 4sin³x = 0
<=> sin³x = 0 hoặc -6cos²x + 1 = 0
<=> sinx = 0 hoặc 3(1+cos2x) = 1
 
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