Text: ISBN Codes. Every book has an International Standard Book Number (ISBN) code. This is a 10-digit code indicating the book's publisher and title. The tenth digit is a check digit satisfying (d1, d2, ..., d10) · (10, 9, ..., 1) ≡ 0 (mod 11). One problem is that d10 might have to be a 10 to make the inner product zero; in this case, 11 digits would be needed to make this scheme work. Therefore, the character X is used for the eleventh digit. So ISBN 3-540-96035-X is a valid ISBN code. (a)

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Determine whether the check digit of the ISBN-10 for this textbook (the eighth edition of Discrete Mathematics and Its Applications) was computed correctly by the publisher. The United States Postal Service (USPS) sells money orders identified by an 11 -digit number x1 x2 …x11 . The first ten digits identify the money order; x11 is a check digit that satisfies x11=x1 x2 ⋯ x10 9 . | Numerade