sin^4x + cos^4x = sin2x - 1/2
<=> (sin^2x + cos^2x)^2 - 2sin^2x.cos^2x = sin2x - 1/2
<=> 1 - 1/2.(2sinx.cosx)^2 = sin2x - 1/2
<=> 2 - (2sinx.cosx)^2 = 2sin2x - 1
<=> 2 - sin^2(2x) = 2sin2x - 1
<=> sin^2(2x) + 2sin2x - 3 = 0
<=> (sin2x - 1)(sin2x + 3) = 0
<=> sin2x - 1 = 0 hoặc sin2x + 3 = 0, loại
<=> sin2x = 1
<=> 2x = π/2 + k2π, k nguyên
<=> x = π/4 + kπ, k nguyên
