1b) (2cosx - 1)(sinx + cosx) = 1
<=> 2sinxcosx + 2cos^2(x) - sinx - cosx = 1
<=> sin2x + 2cos^2(x) - 1 = sinx + cosx
<=> sin2x + cos2x = sinx + cosx
<=> sin(2x + π/4) = sin(x + π/4)
<=> 2x + π/4 = x + π/4 + k2π
hay 2x + π/4 = π - x - π/4 + k2π
<=> x = k2π hay 3x = π/2 + k2π
<=> x = k2π hay x = π/6 + k2π/3
Vậy...