`a)`

`sin 3x+sin x=0`

`<=>` `2sin 2xcosx=0`

`<=>` `sin 2x. cosx=0`

`<=>` `[(sin 2x=0),(cosx=0):}`

`<=>` `[(2x=kpi),(x=pi/2+kpi):}`

`<=>` `[(x=(kpi)/2),(x=pi/2+kpi):}\quad (k  in ZZ)`

`b)`

 `cos^2 3x+sin^2 2x=1`

`<=>` `cos^2 3x=1-sin^2 2x`

`<=>` `cos^2 3x=cos^2 2x`

`<=>` `(1+cos 6x)/2=(1+cos 4x)/2`

`<=>` `cos 6x=cos 4x`

`<=>` `[(6x=4x+k2pi),(6x=-4x+k2pi):}`

`<=>` `[(2x=k2pi),(10x=k2pi):}`

`<=>` `[(x=kpi),(x=(kpi)/5):}\quad (k in ZZ)`

`c)`

`sin(cosx)=0`

`<=>` `cosx=kpi, k in ZZ`

Để PT trên có nghiệm `<=>` `-1<= kpi<= 1<=>k=0`

`=>` `cosx=0`

`<=>x=pi/2+kpi, k in ZZ`