1: =>cosx=căn 3/2
=>x=pi/3+k2pi hoặc x=-pi/3+k2pi
2: =>cos2x=1/3
=>2x=arccos(1/3)+k2pi hoặc 2x=-arccos(1/3)+k2pi
=>x=1/2arcos(1/3)+kpi hoặc x=-1/2arccos(1/3)+kpi
3: =>cos(x-pi/3)=-căn 2/2=3/4pi
=>x-pi/3=3/4pi+k2pi hoặc x-pi/3=-3/4pi+k2pi
=>x=13/12pi+k2pi hoặc x=-5/12pi+k2pi
4: =>cos(2/5pi-x/2)=-căn 3/2
=>2/5pi-x/2=5/6pi+k2pi hoặc 2/5pi-x/2=-5/6pi+k2pi
=>x/2=-13/30pi-k2pi hoặc x/2=37/30pi-k2pi
=>x=-13/60pi-4kpi hoặc x=37/60pi-4kpi
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