giải phương trình:
a) 2sin2x+ sinx= 0
<=>4sinx.cosx+sinx=0
sinx(4cosx+1)=0
sin x=0; x=kπ (k€z)
cosx=-1/4; x=±arccos(-1/4)+k2π

b) sinx+ cos3x= 0
sinx=-cos3x=-sin(π/2-3x)=sin(3x-π/2)
x1=3x1-π/2+k2π
x1=π/4+kπ
x2=π-(3x2-π/2)+t2π
x2=3π/8+tπ/2(t€z)

c) sinx+ 2cosx= 0
cosx =0 ko phai nghiem
<=>tanx+2=0
x=arctan(-2)+kπ

d) 2sin^2.3x=1
sin(3x)=±√2/2
3x=±π/4 +kπ
x=±π/12+kπ/3

e) cos2x= 2cosx-1
<=>(1+cos2x)-2cosx=0
(2cos^2x)-2cosx=0
2cosx.(cosx-1)=0
cosx=0; x=π/2+kπ
cosx=1; x=t2π
(k,t€Z)

2sin2x+sinx-3=0