y= 0.5(1+cosx)
y'=-0.5(sinx)/2=0.5cos(x+π/2)
y"=-0.5(cosx)=0.5cos(x+π)
y"'=0.5(sinx)=0.5cos(x+3π/2)
y(4)=0.5(cosx)=0.5cos(x+2π)
.
y(n)=0.5cos(x+nπ/2)