tan(2x + 1) + cotx = 0 (1)
Đk:
+) cos ( 2x + 1) # 0
<=> 2x +1 # (π/2) + kπ
<=> x # (π/4) + (-1/2) + kπ/2 
+) sin x # 0 <=> x # kπ
PT (1) <=> (sin(2x + 1)/cos(2x + 1)) + cosx/sinx = 0 
<=> sin(2x + 1) sinx + cos(2x + 1) cosx = 0 
<=> cos (2x + 1 - x) = 0 
<=> x + 1 = (π/2) + kπ 
<=> x = -1 (π/2) + kπ (k ∈ Z) ( TMĐK)