sin765=sin(720+45)=sin45=V2/2
sin19π/6=sin(3pi +pi/6)=-sinpi/6=-1/2
2cosx/2=1+cosx
2cosx/2=1+cos2x/2
2cosx/2=2cos^2 x/2
cos^2 x/2-cos x/2=0
cos x/2(cos x/2-1)=0
cos x/2=0 cos x/2=1
x/2=pi/2+pi n x/2=2pi k
x=pi+ 2pi n x=4pi k
n.k eZ
