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sin^23x = sin^22x + sin^2x
tuong duong sin^2x + sin^22x - sin^23x =0
tuong duong 1-cos2x/2 + 1-cos4x/2 + 1-cos6x/2 = 0
tuong duong 1-cos2x + 1-cos4x - 1-cos6x = 0
-cos2x - cos4x -cos6x + 1 =0
cos 2x + cos6x -cos4x + 1 = o
sau do ban bien doi tong thanh tich cos2x + cos6x la ra
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