sinx+sin2x+sin3x+sin4x+sin5x+sin6x=0
<=> sinx +sin6x + sin2x +sin5x + sin3x+sin4x = 0
<=> 2sin(7x/2).cos(5x/2) + 2sin(7x/2).cos(3x/2) + 2.sin(7x/2).cos(x/2) = 0
<=> 2sin(7x/2)[cos(5x/2) + cos(3x/2) + cos(x/2) = 0
<=> 2sin(7x/2)[2cos(2x).cos(x/2) + cos(x/2)] = 0
<=> 2sin(7x/2).cos(x/2)[2cos(2x) + 1] = 0
<=>
{ sin(7x/2) = 0 => 7x/2 = kπ => x = k2π/7
{ cos(x/2) = 0 => x/2 = (2k+1)π/2 => x = (2k+1)π
{2cos(2x) + 1 <=> cos(2x) = -1/2 = cos(2π/3) => x = ± π/3 + k2π
Chúc anh chị học tốt nhé!