zhongxan New member Xu 0 30/6/11 #2 2sinxcos2x + 2sinxcos4x + 2sinxcos6x = 2cos4xsin3x sinx(cos2x + cos4x + cos6x) = cos4xsinx(3 - 4(sinx)^2) (*) neu sinx=0 thi dieu tren hoan toan dung neu sinx khac 0 thi (*) tuong duong voi cos2x +cos6x = cos4x(2-4(sinx)^2) 2cos4xcos2x = 2cos4xcos2x (dpcm)
2sinxcos2x + 2sinxcos4x + 2sinxcos6x = 2cos4xsin3x sinx(cos2x + cos4x + cos6x) = cos4xsinx(3 - 4(sinx)^2) (*) neu sinx=0 thi dieu tren hoan toan dung neu sinx khac 0 thi (*) tuong duong voi cos2x +cos6x = cos4x(2-4(sinx)^2) 2cos4xcos2x = 2cos4xcos2x (dpcm)