How do you find the phase shift in a Fourier Series?

Specifically in electricity, I am unsure how to find the phase shift of a Fourier Series that represents a current. The period is 20ms. The particular problem I have a Co =3.2mA and a Cn magnitude of 8/(n*pi)sin(4*pi/5). When I want to put in in the Cn complete form I get to 3.2 + (sigma) 8/(n*pi)sin(4*pi/5)cos(n(2*pi)/(20E-3)t + ????) = i(t) Where the ???? is the phase shift I am unsure of. I have tabled the magnitudes of n= 1 to 6 and I realize there is a magnitude of 0 at 5. I think the phase can be in a closed form because the magnitude is purely real but have no idea where to go from there.