giúp em với ạ......... 1) giải phương trình: a) cos2x-sinx=căn 3.(sin2x-cosx) b) cos bình x + cos bình 2x + cos bình 3x + cos bình 4x = 2 2) tìm m để phương trình cos 2x - (2m - 1)cos x - 2m = 0 có nghiệm 3) tìm giá trị lớn nhất giá trị nhỏ nhất của hàm số y = (1 + cosx )/(sinx + cosx - 2)

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