http://repository.upi.edu/1605/6/S_ADP_0705056_Chapter3.pdf
http://repository.unpas.ac.id/30307/4/bab%203.pdf
https://simdos.unud.ac.id/uploads/file_pendidikan_1_dir/46052a087c2458d7c6ca400c0139c7a7.pdf
https://eprints.umm.ac.id/35344/4/jiptummpp-gdl-riscamitaa-48836-4-babiii.pdf