Looks like angle sum identity. cos(x+y) = cos(x)*cos(y) - sin(x)*sin(y) (always) ~ cos(x)*1 - sin(x)*y (if y small) Can't s

Looks like angle sum identity. cos(x+y) = cos(x)*cos(y) - sin(x)*sin(y) (always) ~ cos(x)*1 - sin(x)*y (if y small) Can't speak for the validity of small angle assumptions though.