f(x)=√2cos(x/2)-(a-1)sin(x/2)
f(-x)=√2cos(x/2)+(a-1)sin(x/2)
f(x)=f(-x)
a-1=0
a=1
f(x)=√2cos(x/2)
T=2π/ω=2π/(1/2)=4π