Here,
cosh^2(x) = 1/2(1 + cosh(2x)
L.H.S. = cosh^2(x) = (coshx)^2 = [(e^x+e^-x)/2]^2 =
= (e^2x+e^-2x +2)/4 = [(e^2x + e^-2x)/2] / 2 + 1/2 =
= 1/2(1 + cosh(2x) = R.H.S. >=========================< Q . E . D.
ALSO,
cosh^2 (2x) = 1/2(1 + cosh(4x) .... you are correct.
Hence,
cos^2x =...