cos2x + (1 + 2cosx)(sinx-cosx) = 0<=> (cos2x - sin2x) - (1 + 2cosx)(cosx - sinx) = 0<=> (cosx - sinx) (cosx + sinx - 1 - 2cosx) = 0
* cosx - sinx = 0 <=> 1 - tanx = 0 (ĐK: cosx khác 0)
<=> tanx = 1 <=> x = pi/4 + k.pi
* sinx - 1 - cosx = 0 <=> (sinx - cosx)2 = 1
<=> (sin2x - 2sinxcosx + cos2x) = 1
<=> 2sinxcosx = 0
<=> sinx = 0 <=> x = k.pi
cosx = 0 <=> x = pi/2 + k.pi