Rujukan Pekali

  • Sabah Al-hadad and C.H. Scott (1979) College Algebra with Applications, halaman 42, Winthrop Publishers, Cambridge Massachusetts ISBN 0-87626-140-3 .
  • Gordon Fuller, Walter L Wilson, Henry C Miller, (1982) College Algebra, edisi ke-5, halaman 24, Brooks/Cole Publishing, Monterey California ISBN 0-534-01138-1 .
  • Steven Schwartzman (1994) The Words of Mathematics: an etymological dictionary of mathematical terms used in English, halaman 48, Mathematics Association of America, ISBN 0-88385-511-9.