Richard N. Aufmann, Joanne Lockwood, Introductory Algebra: An Applied Approach, Publisher Cengage Learning, 2010, ISBN 1439046042, 9781439046043, page 78 14 Temmuz 2014 tarihinde Wayback Machine sitesinde arşivlendi.
William L. Hosch (editor), The Britannica Guide to Algebra and Trigonometry, Britannica Educational Publishing, The Rosen Publishing Group, 2010, ISBN 1615302190, 9781615302192, page 71 2 Aralık 2013 tarihinde Wayback Machine sitesinde arşivlendi.
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Richard N. Aufmann, Joanne Lockwood, Introductory Algebra: An Applied Approach, Publisher Cengage Learning, 2010, ISBN 1439046042, 9781439046043, page 78 14 Temmuz 2014 tarihinde Wayback Machine sitesinde arşivlendi.
William L. Hosch (editor), The Britannica Guide to Algebra and Trigonometry, Britannica Educational Publishing, The Rosen Publishing Group, 2010, ISBN 1615302190, 9781615302192, page 71 2 Aralık 2013 tarihinde Wayback Machine sitesinde arşivlendi.