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2sin2x-cos2x=7sinx+2cosx-4
\Leftrightarrow [TEX]4sinxcosx + 2sin^{2}x - 1 = 7sinx + 2cosx - 4[/TEX]
\Leftrightarrow [TEX]4sinxcosx + 2sin^{2}x + 3 - 7sinx - 2cosx = 0[/TEX]
\Leftrightarrow [TEX](2sin^{2}x - 7sinx + 3) + 2cosx( 2sinx - 1) = 0[/TEX]
\Leftrightarrow [TEX](2sinx - 1)(sinx - 3) + 2cosx(2sinx -1) = 0[/TEX]
\Leftrightarrow [TEX](2sinx - 1)(sinx - 3 + 2cosx ) = 0[/TEX]
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