Question

Which of the following are identities?
a. cos(x+y) + cos (x+y)= 2cos x cosy
b. sin(x+y) + sin(x+y) = 2cos x sin y
c. cos(xty) - cos(x-y) = 2cosx cosy
d. tan(x-pi) = tanx
e. (1-tan^2x)/(2tan x)^2 = 1+sin2x
f. (sinx - cosx)^2 = 1 +sin2x
g. (sin 8x = 2sin 4x cos 4x
h. (sin x + sin 5x)/(cos x + cos5x) = tan3x