[Toán 11] Phương trình lượng giác

T

thienlong233

cos2x+2cos^2(2x)-1+4cos^3(2x)-3cos2x = cos2x.(cosx.cos3x)+2.
cos2x+2cos^2(2x)-1+4cos^3(2x)-3cos2x = 1/2.cos2x(cos4x+cos2x)+2.
cos2x + 2cos^2(2x)-1+4cos^3(2x)-3cos2x= 1/2.cos2x(2cos^2(2x)-1+cos2x)+2.
<=> 2cos^3(2x) + cos^2(2x) - cos2x - 2 =0.
<=> cos2x = 1.
 
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