Stuttgart Kultusminsterium Stuttgart von der Landesanstalt fur Erziehung und Unterricht 1959
Buku Teks
Kamke, E.
Dover Publications INC, 1950
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Terman, Frederick E. (Frederick Emmons)
McGraw-Hill, 1982
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Webster, Frederick E.
Prentice-Hall, 1972
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Prentice-Hall, 1997
Buku Teks