https://repository.unair.ac.id/54545/2/ff%20fk%2001%2016.pdf
http://repository.um-surabaya.ac.id/5659/3/BAB_II.pdf
http://eprints.undip.ac.id/43750/3/Damayanti_Ika_Prasanti_G2A009057_Bab2KTI.pdf
http://repository.poltekeskupang.ac.id/1432/1/001.KTI%20R.A.SEO.pdf
