f(x)=sin(2x+a)=sin(2x)cosa+cos(2x)sina
f(-x)=sin(-2x+a)=-sin(2x)cosa+cos(2x)sina
f(x)=f(-x)
cosa=0
a=pai/2+k pai k属于Z
f(x)=sin(2x+pai/2+k pai)
f(x)=sin(2x+a)=sin(2x)cosa+cos(2x)sina
f(-x)=sin(-2x+a)=-sin(2x)cosa+cos(2x)sina
f(x)=f(-x)
cosa=0
a=pai/2+k pai k属于Z
f(x)=sin(2x+pai/2+k pai)