Chứng minh: sin6x*sin4x -sin15x*sin13x + in19x*sin9x=0

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1. Ta có:

    `\qquad sin6xsin4x-sin15xsin13x+sin19xsin9x`

   `=1/2[cos(6x-4x)-cos(6x+4x)]-1/2[cos(15x-13x)-cos(15x+13x)]+1/2[cos(19x-9x)-cos(19x+9x)]`

   `=1/2(cos2x-cos10x)-1/2(cos2x-cos28x)+1/2(cos10x-cos28x)`

   `=1/2cos2x-1/2cos10x-1/2cos2x+1/2cos28x+1/2cos10x-1/2cos28x`

   `=0`

   Vậy: `sin6xsin4x-sin15xsin13x+sin19xsin9x=0`

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