1.DK: sin2x va cos2x khac 0 
<=> x ≠ kπ/4 
(1-cos2x)/(sin2x)^2 = 1 / sin²2x - cos2x/sin²2x = 1 / sin²2x - cot2x / sin2x 
==> bt<=> 1 + cot2x = 1 / sin²2x - cot2x / sin2x 
<=> 1 / sin²2x - 1 - cot2x / sin2x - cot2x = 0 
<=> cot²2x - cot2x - cot2x / sin2x = 0 
<=>cot2x(cos2x/sin2x - 1 - 1/sin2x) = 0 
+) voi: cot2x = 0 <=> x = π/4 + kπ/2 (k thuoc Z) 
+) voi: cos2x/sin2x - 1 - 1/sin2x = 0 
<=> (cos2x - sin2x - 1) = 0 
<=> 1/√2cos2x - 1/√2sin2x = 1/√2 
<=>sin(π/4 - 2x) = sin(π/4) 
<=> π/4 - 2x = π/4 + k2π hoac π/4 - 2x = 3π/4 + k2π 
<=> x = -kπ hoac x = -π/4 - kπ (k thuoc Z) 
ket hop lai ta co nghiem pt: x = π/4 + kπ/2 ; x = kπ ( k thuoc Z)