*B . Williamson " Subtangent and Subnormal " and " Polar Subtangent and Polar Subnormal " in " An elementary treatise on the differential calculus " ( 1899 ) p 215, 223 Google books
2.
*B . Williamson " Subtangent and Subnormal " and " Polar Subtangent and Polar Subnormal " in " An elementary treatise on the differential calculus " ( 1899 ) p 215, 223 Google books
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